Saturday, December 22, 2012

Low-Side MOSFET Drive Circuits and Techniques - 7 Practical Circuits


In many circuits, it is necessary to use MOSFETs for switching. In many cases, the MOSFET drive signals are generated by microcontrollers. In other cases, they are generated by ICs – PWM controllers, timers or any IC in fact. However, MOSFETs cannot always just be connected to the drive signal and be expected to work properly. Due to the construction of the MOSFET, driving it is not the simplest of tasks, especially for beginners. There are many users who regularly ask for help on MOSFET drive related issues or problems on different blogs, websites and forums. So, here I will show some MOSFET drive techniques/methods for MOSFETs configured as low-side switches.

Before I head on to MOSFET drive, let me just tell you what a low-side switch is, in case you don't know. When the MOSFET (that you're using as a switch) sinks current, it is a low-side switch. The load will be between the drain and +V supply. The source will be connected to ground. Gate will be driven with respect to ground.

Let me start with N-channel Power MOSFETs. One thing to keep in mind is that these MOSFETs are fully turned on when the voltage at the gate with respect to the source (VGS) is > 8V. So while at voltages less than 8V, the MOSFET will be on (as long as VGS > VGS(threshold) ), the MOSFET won’t be fully turned on. It will operate in the linear region and there will be very high losses.

Now let’s go to Logic Level MOSFETs. These MOSFETs tend to have much lower VGS (threshold) and thus are usually fully turned on with VGS of 4V to 5V. One downside to Logic Level MOSFETs is that they usually have higher on-state resistances compared to similarly rated Power MOSFETs. So, this would mean higher conduction losses for the same load.

Another thing to remember is the gate capacitance. So, when the MOSFET is to be turned on or off, the gate must be driven high or low with sufficient current quickly enough so as to charge or discharge the gate capacitance quickly enough so that the MOSFET spends minimal time in the linear region and is quickly turned fully on or off. This is true especially for high speed switching when time period is small. However, in low frequency applications, this may not be a big problem as, even if the MOSFET spends some time in the linear region, it will spend the (remaining) majority time fully on and thus the small amount of time spent in the linear region will not cause much of a problem.

Let’s first consider the simplest situation: a MOSFET is driving a load and is periodically turned on and off to turn the load on and off respectively. How long do I mean by periodically? Well, it really depends on the application. But for this situation, the period between turning on and off may be very long or very short but will be at least half a second. A few examples:

  • The MOSFET could be driving a DC fan that will be turned on, kept on for 2 hours and then kept off for about an hour and turned on and off in few hour intervals.
  • The MOSFET is driving a DC lamp that will stay on for the entire night and will only be turned off in the morning and kept off during the entire day.
  • The MOSFET is driving a heating element through which current passes (the heater is on) for about 16 hours a day.
  • The MOSFET is driving an IR LED in a remote transmitter unit. The LED is turned on and off frequently – every few seconds.

So, the above examples should clarify what I meant as “periodically” in the preceding paragraph.
In such situations, MOSFET drive is simplest – apply a high enough voltage to the MOSFET gate and the MOSFET will be on; ground the MOSFET gate and the MOSFET will be off. Quick charging and discharging of the MOSFET gate capacitance are always desirable. But in this situation, the charging and discharging need not be very quick as frequency isn’t high.
Here are a few circuits that can easily be used.




 Fig. 1  (Click on image to enlarge it)


Fig. 2  (Click on image to enlarge it)


Fig. 3  (Click on image to enlarge it)


Fig. 4  (Click on image to enlarge it)


In each of these circuits Vin is the input drive signal to the MOSFET. +V_DRIVE is the voltage supply to drive the MOSFET. When the MOSFET is on, VGS = +V_DRIVE. When the MOSFET is off, VGS = 0. MOSFETs have a maximum rated VGS. For most MOSFETs, this is 20V. For some it is higher up to 30V. There are MOSFETs (usually Logic Level) which have a lower maximum rated VGS. It is always best to check the datasheet for the specific MOSFET being used.


Fig. 1 shows an inverting driver. Here, the MOSFET is turned on when Vin is low and turned off when Vin is high. When the MOSFET is on, the load is on and vice versa.

Fig. 2, Fig. 3 and Fig. 4 show non-inverting drivers where the MOSFET is turned on when Vin is high and the MOSFET is turned off when Vin is low.

In Fig. 2, the MOSFET turns off almost instantaneously when Vin is zero. This is because when Vin is zero, Q3 is off and Q5 is on. Q5 pulls the gate low, discharging the gate capacitance. However, the MOSFET turn-on is not instantaneous. When Vin is high, Q3 is on and Q5 is off. So, current flows through R14 and charges the MOSFET gate. Here turn-on time is dictated by R14. However, when I say long turn-on time, I am talking in the order of tens or hundreds of microseconds to milliseconds at max. Thus this circuit can easily be used in any of the situations depicted in the four examples above or any similar situation. This driver cannot be used when switching is done in the order of microseconds where both turn-on and turn-off must be very quick – almost instantaneous.

In Fig. 3, the MOSFET turns on almost instantaneously when Vin is one. When Vin is one, Q6 is on and so Q8 is on and thus VGS = +VDRIVE and so the MOSFET turns on. However, when Vin is zero (and so Q6 and Q8 are off), the MOSFET gate discharges through R18. So, gate discharge is slow and turn-off time is dictated by R18. However, this “slow” time is in the same order as mentioned above and so can be used in any of the situations depicted in the four examples above or any similar situation. Similarly, this driver cannot be used when switching is done in the order of microseconds where both turn-on and turn-off must be very quick – almost instantaneous.

In Fig. 4, both turn-on and turn-off are very quick. This driver can be used for higher frequencies than can the other three.

If the MOSFET is a Power MOSFET, +V_DRIVE should be at least 8V. A voltage commonly used is 12V. +V_DRIVE should be less than VGS(max) as specified in the MOSFET datasheet. Some “headroom” should be used. So, if VGS(max) is specified as 20V in the MOSFET datasheet, it is good practice to use lower than 18V. 12V and 15V are commonly used for Power MOSFET gate drive. For Logic Level MOSFETs, 5V is most common because the reason a Logic Level MOSFET is usually used instead of a Power MOSFET is that the Logic Level MOSFET can be driven from 5V. In the above shown circuits, the MOSFET can be either a Logic Level MOSFET or a Power MOSFET. +V_DRIVE must be selected accordingly.

Each of the above four circuits requires only a minimum voltage of 0.7V to fully turn on the MOSFET (+V_DRIVE must be selected properly). Each of the above circuits can also be used to drive either Logic Level MOSFETs or Power MOSFETs.

Here are a few more circuits that I’ll discuss below.




Fig. 5  (Click on image to enlarge it)

Fig. 6  (Click on image to enlarge it)


 
Fig. 5 and Fig. 6 both show non-inverting drivers.

The driver in Fig. 5 has the advantage that, while being simple, it can both quickly turn on and turn off the MOSFET. In the drivers in Fig. 1, Fig. 2, Fig.3 and Fig. 4, when the MOSFET is on, VGS = +V_DRIVE. The MOSFET is turned no when Vin is at least 0.7V. So, that means that when Vin is higher than 0.7V, VGS = +V_DRIVE. So, for example, if +V_DRIVE is 12V, VGS will be +12V, when Vin = 3.3V, 5V, 9V and so on.

However, in the drivers shown in Fig. 5 and Fig. 6, when the MOSFET is on, VGS = Vin – 0.7V. So, that means that in Fig. 5, even if +V_DRIVE is +12V, when the MOSFET is on, VGS will only be equal to Vin – 0.7V and not +V_DRIVE. So, for Fig. 5 and Fig. 6, if +V_DRIVE is +12V and Vin = +5V, VGS will equal +4.3V when the MOSFET is on.

That is the disadvantage to these circuits – that the MOSFET VGS will depend on the magnitude of Vin. However, there are situations where these disadvantages do not matter.

If the MOSFET is a Logic Level MOSFET that is to be driven by a microcontroller, then the driver in Fig. 5 can be used quite easily. When Vin = +5V (typical output of a microcontroller), the MOSFET will be turned on and VGS = +4.3V. Remember that +4.3V is enough to fully turn on the MOSFET. When Vin = 0, VGS = 0 and the MOSFET will be off.

You may, then, ask why a driver is needed. You may simply drive the microcontroller directly from the microcontroller. And well, you can. For low speed applications. So, in the four situations previously described, the microcontroller can directly drive the Logic Level MOSFET. The problem lies where the microcontroller output voltage will drop when supplying the current needed to quickly turn the MOSFET on and off when a large current will be required. So, the MOSFET may not fully turn on. In that case, the driver shown in Fig. 5 can be used to ensure that the MOSFET can be driven with sufficient current.

The driver in Fig. 6 can be used when the control chip can only source current but cannot sink current. One example is the SG3524 PWM controller configured to source current. The 0.7V drop is not a problem since the outputs of the SG3524, when high, go up to near the supply voltage. So, even after the 0.7V drop, the voltage is sufficient to fully turn the MOSFET on.

The driver in Fig. 5 is commonly used to drive a MOSFET from a PWM controller. Usually the outputs of PWM controllers go all the way up to the supply voltage (usually 12V, 15V, etc). So, this driver can easily be used, because even after the 0.7V drop, the voltage is high enough to fully turn on a MOSFET. The transistors can also supply high currents. This type of driver based on such a combination of transistors is called a totem-pole driver.

Till now, I’ve only talked about low frequency applications. Now let’s move on to high frequency applications.

One thing that must be considered in high frequency applications is the MOSFET drive current. When frequency is high, time period is low. So, the MOSFET must be turned on and off in a short time to avoid high losses. The turn-on and turn-off times must be as short as possible to ensure that the MOSFET will be in the linear region for minimum time.

Thus it is natural that to minimize gate charge time, current must be maximized. So, the quicker the gate must be charged or discharged, the higher the charge or discharge current should be. The drivers in Fig.  4, Fig. 5 and Fig. 6 can be used for high-frequency MOSFET drive.

I’ve covered some discrete drivers here based on transistors. However, I always recommend using dedicated driver chips for driving Power MOSFETs in high-frequency applications. Low-side drivers are easily available at low prices. One widely available low-cost low-side driver (that I often use) is the TC427.

The circuit using a TC427 is very easy. The supply voltage has to be provided to the supply pins. Then, all you need to do is provide the input signal and just drive the MOSFET from the TC427 output. The TC427 has 2 drivers. So you can use the TC427 to drive 2 MOSFETs from 2 different input signals. Here is the circuit diagram:

 Fig. 7


 
As you can see, the circuit is very simple. +V is the supply voltage. MOSFET drive voltage swings up to about VDD - 0.025V when MOSFET is to be driven high and down to +0.025V (can be assumed to be zero as this is way below MOSFET VGS(threshold) when MOSFET is to be driven low.

Download the TC427 datasheet from here:

There are numerous ways to drive a MOSFET in a non-isolated low-side configuration. I’ve shown a few circuits using transistors and one using a dedicated driver.

You can find loads of driver chips and the datasheets should show the circuit diagrams. Here I have only covered low-side non-isolated drive. I shall soon make another tutorial where I cover high-side drive and isolated drive.

I hope that this tutorial has helped you. Let me know your comments and feedback.

75 comments:

  1. What are the values if resistances used in figure 4?

    ReplyDelete
  2. You can try with a range of values for R28 and R29. 1k is okay. For low frequencies, even 10k may be okay.

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    1. This comment has been removed by the author.

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    3. If R28,R29 are too low (i.e. values comparable to R23) then the resistors will cause loading of the previous stage. This will cause the voltage at bottom point of R23 to hover somewhere between 0 and V+ rail. If this happens the output npn-pnp pair will conduct simultaneously (in linear mode) and cause heating up and possible burn-out of the pair.
      On the other hand using excessively high values will cause a problem with the switching of the MOS gate, because the maximum available output current from the npn-pnp pair will be lesser (lesser maximum base current available etc.). With R28,R29 values of 10K and R22 of 4K7 with off-the-shelf npn and pnp transistors (BC547,BC557) the rise/fall times at the output MOS gate is around 250nS for 3A load and 6V supply, for a IRFZ44N MOSFET.
      This means that it should be useable to upto around 1kHz (rise/fall time << 1/10th of waveform period), Practically this is safe to use upto around 2kHz or so at around 4A and 6V drive voltage available, provided you've a reasonable heatsink on the MOS (For IRFZ44N).
      You agree Tahmid?

      Delete
    4. Yes, you have to ensure that R28 and R29 are high enough that you don't cause loading of the previous stage. You'll have to ensure that you can keep the transistor in saturation mode, so that the collector voltage of Q9 is always V_DRIVE or zero but not in between or you risk turning both transistors on.

      You can also improve this by increasing transistor drive current.

      I'll run some simulation tests to get some numbers.

      Delete
  3. one disadvantage is there in fig:4 .

    if Vbe of Q11=8v and vbe of Q12=8v .so at that time both transister become on , may occure damage .
    but vbe of both transistor should not come 8v ,and which have only 0v or 12v ,anyway if it happend then transister bcme damage .Right?

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  4. Yes, what you have said is correct. If +V_DRIVE is +12V and voltage at collector of Q9 is 8V, then there will be cross-conduction : Q11 and Q12 will both be on and there'll be a short-circuit. However, there is no scope for the collector of Q9 to be at 8V unless R23 is very small, which it shouldn't be. With 1k as shown, there'll be no problem.

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  5. if Vin is <0.7v then collector of Q9=12v , if >0.7v then it is 0v . ok am agree .
    But if Vin is =0.7v what will happen . and we can't say it may one of the state in 0 or 12 .
    i think it will be in between 0 and 12 [so both transitor become on Q11 and Q12]. because transister haven't any sharp cut of voltage for vbe=0.7v, and this value will vary for each transtr.
    So for a deep thinking Pure designer will not accept this circiut[fig ;4]. right tahmid?

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  6. For simplicity's sake it has been considered that VBE = 0.7V but you must keep in mind that this is a property of the transistor and may vary depending on the transistor, although it should usually be around 0.7V (or 0.6V).

    At certain voltages, it may cause problem. But, the input shouldn't ever be 0.7V since it will come from an IC or microcontroller, whose output when high is never 0.7V but nearer the supply voltage, due to which this problem is never faced practically.

    And that is why, this circuit is accepted and also used in many commercial circuits, even in India.

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  7. agree thamid thanks .
    can you give me your email id?
    my id : ismailpp2008@gmail.com

    ReplyDelete
  8. Hi,

    My email ID is inferno-rage (at) hotmail.com

    ReplyDelete
  9. hi thamid

    in above u r mentioned about low side driver and its relatives, But Can you explain how its related to High side driver circuit?. with simple example . And pls explain Which mos is good for low side and which one is for high side Ie: n- channel /p-channel.

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  10. I will soon write up an article about high side drive. Currently I am quite busy. Once I'm free, I'll write some more articles/tutorials.

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  11. I've written an aricle on high-side and low-side drive using IR2110. Perhaps doubts regarding high-side drive, at least those regarding IR2110, will be cleared if you check it out:

    Using the high-low side driver IR2110 - explanation and plenty of example circuits:
    http://tahmidmc.blogspot.com/2013/01/using-high-low-side-driver-ir2110-with.html

    Regards,
    Tahmid.

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  12. Hi Tahmid, Its Mehdi from dhaka, I made a circuit for solar charge controller using N-Mosfet and some drive components. It works for 12 volt system but problem when drive for 24 volt system. If I could sent it for you, then may be you can help me. Do I have your email ID or phone no? My email : mehdi98_here@yahoo.com, cell: 01713112249
    Best Regards
    Mehdi

    ReplyDelete
    Replies
    1. Hi,

      My email address is inferno-rage (at) hotmail.com

      However, I recommend you upload the schematic to a file storage server such as rapidshare. Everyone can benefit if the discussion goes on here.

      Regards,
      Tahmid.

      Delete
  13. Hi mehdi and thahmid ,
    i have same problem when i am using 24v for charger , i have just convert 24v to 12v using 7812 ,
    But 7812 getting damage instantaneously.
    please go through this link, i have updated that issue in edaboard :

    http://www.edaboard.com/thread281331.html#post1204711

    ReplyDelete
    Replies
    1. In situations where there is a large voltage gap between input and output, I always recommend using switching regulators. Some common regulators include the LM2575, L4971 (this one's cool because it's in an 8-pin PDIP package), L4962, etc.

      ___________________________________________________________________________________


      I've replied to the edaboard thread as well.

      ___________________________________________________________________________________


      Regards,
      Tahmid.

      Delete
  14. Thanks Tahmid for your reply. I got my problem. It was nothing but the ground reference. I tied both battery positive and solar positive together and tried to switch on ground lines. Battery connected to the mosfet drain and solar connected to the source. As driving for 12V system both voltage level are same but when driving for 24 volts there's the problem because I supplied the drive power from 12V battery with ground from battery. Now I drive mosfet gate with reference to solar ground and no problem. I will upload the circuit very soon.
    Thanks you guys again.
    Mehdi

    ReplyDelete
    Replies
    1. Glad to hear that you've fixed it!

      Regards,
      Tahmid.

      Delete
  15. Hi Tahmid,
    The MOSFET IRFZ44V heated much when I used this in Buck converter (during low pulse only). Its gate volt is 4.64. What might be the problem?

    Thanks.
    Mitz

    ReplyDelete
    Replies
    1. Is 4.64V the average voltage or "peak voltage level"? You should upload the schematic, especially of the driver. Upload it to an online file storage site, such as rapidshare. Make sure you make the file "public".

      Regards,
      Tahmid.

      Delete
  16. What is the purpose of having 10k resistors to pull down mosfet driver input ? Are they required ? Also may I ask what kind of resistors and values you are using between mosfet driver and mosfet gate ? I see on schematic you posted 10 ohm resistor, but how you found that particular value ?
    Sorry, one more question if you don't mind : what kind of capacitors can be used for decoupling mosfet driver ? Some datasheet propose film capacitors but MKP types are rated for much higher voltages and are big, can I use tantalum capacitor here ? (I'm trying to use smd components)

    ReplyDelete
    Replies
    1. The 10k pull down resistors have been used to ensure that no input pin is floating, so thatstray noise is not picked up.

      For the gate resistance, usually values between 4.7 ohm and 22 ohm can be used. 10 ohms is a common value that can work. For high frequencies (100kHz and above), you may need to reduce that to about 6.8 ohms. For more detail regarding the gate resistance, refer to this:

      http://www.edaboard.com/thread221750.html#post943534

      For low capacitance (eg 100nF) decoupling capacitors, ceramic types are the best. Try to use ceramic disc capacitors instead of tantalum capacitors. You should be able to easily get 50V ceramic disc capacitors. Like these:

      http://uk.mouser.com/Passive-Components/Capacitors/Ceramic-Capacitors/Ceramic-Disc-Capacitors/_/N-5g90?P=1z0x83k&Keyword=100nf&FS=True

      Hope this helps.

      Regards,
      Tahmid.

      Delete
  17. jajha kumullah khair,

    it has broaden the door to entering in the power electronics world for the beginners as me

    ReplyDelete
  18. Hello Tahmid I have been following your tutorial all along even in other forums. i am trying to build a 1000 watt inverter to run my fridge but having great problems. I built fig 5 as my driver for irfz44 fets in my dc to dc converter but as u say u need about 8v to fully turn on the fets. I want to know if I should have this voltage on the gate of the fets when there is no load on the output of the h bridge. Please clearlt tell me if this voltage should be there at all times. This is my question I have been trying to get an answer for for the past two months. Thank you

    ReplyDelete
    Replies
    1. Remember that the circuits shown here are for low-side MOSFET drive and so should not be used for H-bridge drive unless you're using them for driving the low-side MOSFETs in the H-bridge.

      To drive the MOSFET fully on (with or without load), the gate of the MOFSET should be at least about 8V higher than the source. However, if you don't have a load, the circuit is incomplete and it doesn't matter if the MOSFET is on or off. But, considering that a load may be applied at any time, since it's an inverter, the voltage on the gate must be present whenever the MOSFET is to be on.

      I hope that answers your question.

      Regards,
      Tahmid.

      Delete
  19. DEAR Tahmid

    i need your help
    i have 4 n-channel igbt (G40N60) , i connected them in H-Bridge
    can i drive them using driver circuit shown in fig.4 ??
    i use pic16f377A to generate squre wave

    ReplyDelete
    Replies
    1. The circuits I've presented here are only for low-side MOSFET/IGBT drive. You can use the driver circuit shown in Fig. 4 only to drive the two low-side IGBTs. However, I recommend you use dedicated high-low side MOSFET/IGBT drivers such as IR2110 to drive all the MOSFETs.

      Refer to this tutorial that I've written:

      Using the high-low side driver IR2110 - explanation and plenty of example circuits

      http://tahmidmc.blogspot.com/2013/01/using-high-low-side-driver-ir2110-with.html

      Regards,
      Tahmid.

      Delete
  20. hello Tahmid keep the good work gooing. I have a question can i use fig 4 to drive a pushpull stage for a transformer base output inverter running at 50hz. Thank u for ur support

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    Replies
    1. Yes, you can. The push-pull stage consists of 2 MOSFETs (or 2 sets of paralleled MOSFETs) that are both low-side. So, you can use that circuit.

      Regards,
      Tahmid.

      Delete
  21. thank u very mucg Tahmid because I tried fig 1 but only got about 6.80v at the output and when I connect it to my pushpull config it just caused the transformer to make a heavy sound and cut the ac volts

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    Replies
    1. What's the frequency of the driving signal and the duty cycle? What's the supply voltage?

      Where does the driving signal come from?

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    2. thank you for your reply the drive frequency is 60hz coming from a sg3524 and i was using 12v to supply the drive circuit in fig 1

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    3. Can you check the output waveform with an oscilloscope and post the pictures?

      If you're using the SG3524 powered off 12V to generate the signals, the voltage you measure shouldn't be more than 6V, as maximum duty cycle should be 50%.

      12V * 0.5 = 6V

      Are you using your voltmeter to measure the voltage at the SG3524 outputs?

      Delete
  22. hello Thamid one more question if I use the tc427 to drive my pushpull output inverter and use a supply volt of 12v for the tc427 would my vgs volt to thefets be also 12v thank u

    ReplyDelete
    Replies
    1. VGS when the MOSFET is driven on would be about 0.025V below 12V (VDD - 0.025V).

      Average voltage registered by the voltmeter would be (VDD - 0.025V) * (Duty Cycle).

      Regards,
      Tahmid.

      Delete
    2. thank you tahmid . I am building this transformer output inverter but the sg3524 is only puting about 3.3v on the gates and as from other blogs u said the fets should be getting about 8 or 10v to fully turn on I am using irf260n so I think I need a drive circuit. Is this correct. Thank you fro ur support

      Delete
    3. If you're using a voltmeter to measure the output from the SG3524, you're only reading an average voltage. The voltage that matters is the amplitude.

      That being said, the SG3524 doesn't have a totem-pole driver, but only either a current source/sink. So, you should use a MOSFET driver anyways when using the SG3524.

      Regards,
      Tahmid.

      Delete
  23. Hi, am designing using proteus 7.7 on a 0.5hp, 415v induction motor drive. I'm making use of 6 1200v IGBT switches but i'm currently having a problem with the drivers available in proteus. I therefore want to know if it's possible to use 600v drivers for these switches.

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    Replies
    1. If your maximum supply voltage is less than about 500V, you should be able to use the 600V drivers. It's the maximum supply voltage that matters when selecting the driver and not the voltage ratings of the IGBTs. You could very well have used 900V IGBTs or even 2000V IGBTs, but the supply voltage would still be the same.

      Regards,
      Tahmid.

      Delete
  24. Hello Tahmid.
    I am using the TC427 to drive the low side N-mosfet from H-bridge.
    The P-mosfet from high side is on. (I am only testing one direction of the bridge)
    When I start, stop the motor the TC427 burned. Now I am using the 4 diodes in the H-bridge
    and the new TC I use is alive.
    It is so easy to burn them?
    The P-mos is IRF4905, the N-mos is IRF3205.
    I will design a high bridge and I am afraid that the TC will get burned easy.
    What may be the cause?
    If you could let me some mail I can send you the circuit.
    Thanks!
    Sebastian.

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  25. Hello,

    I'm curious in R23 in Fig4. If I manage to get the resistance so high (let's say it's much higher than CE junction of Q9 when it's cut off if I can get a hand on), will the collector of Q9 always give only 0V without Vin=5V input?

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    Replies
    1. There needs to be a minimum collector current flowing to ensure proper operation of the transistor and to get 0V at collector when Vin = 5V. The collector current must be greater than ICEO (specified in the datasheet). So this means that you can't pick a resistance (for R23) too close to the resistor of the CE junction, let alone higher. In fact it should be at least 10 times less in order to ensure proper operation. Of course this 10 times isn't a hard set rule, but something you can do to avoid problems.

      Delete
    2. You need to factor into the collector current due to pulling in the PNP base as well.

      Also do remember you need to be able to supply enough current from this stage so that the PNP transistor in the next stage can supply high currents to drive the MOSFET if required.

      Delete
  26. If there is no R25 in Figure4 and the gate voltage of FET become zero, will the discharged charges from Gate-Source internal capacitance damage the FET?

    (cuz zero gate voltage is equal to source voltage which is grounded. Without R25, can we imply that the gate is shorted with the source? or it's just a virtual short circuit and there is no discharged current?)

    ReplyDelete
    Replies
    1. If there is no R25, there should (ideally) be no problem since zero gate voltage, like you said, means that the gate is at source potential and the gate is "shorted" to the source. Even with R25, this is what happens with the transistor pulling the gate low. The importance of R25 is not to turn the MOSFET off but rather to prevent it from accidentally turning on due to the drain-gate parasitic capacitance. Read here: http://tahmidmc.blogspot.com/2012/10/magic-of-knowledge.html

      Delete
  27. Hi Tamid,

    Could you tell me the transistors I could use to turn on my Mosfets. You have labelled Q1,Q3,Q5, etc.
    I want to design a Synchronous Boost Converter for a certain application. By synchronous boost I mean that the diode in the boost converter is replaced by a Mosfet which works as a switch

    ReplyDelete
  28. How i can replace a relay driven by a PIC with a Mosfet.
    The load is 24VAC / 2Amps ?

    ReplyDelete
    Replies
    1. Google 'solid state relay with MOSFET'.

      Delete
  29. Hello, can i use na dc motor after de mosfet?, Ex, in a car, the dc motor is grounded, so de mosfet needs a P Chanel, but i can use a N Chanel with the dc motor after, and a driver transistor ? example circuit? (sorry, i don,t speak english...) Thanks, estevamfreitas@bol.com.br

    ReplyDelete
    Replies
    1. You can put the load between drain and supply voltage and operate the N-channel MOSFET in the low-side to drive the load. If you want to use the N-channel on the high-side, you'll need to use a high-side driver. The choice of driver will be dictated by your required duty cycle, etc.

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  30. I am thinking of using circuit 7.I have a problem.I never understand the ground connections.The output of microcontroller is fed to TC427 as input.Should the ground of micro controller be shorted with the ground of the chip,TC427.

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    Replies
    1. TC427 is a non-isolated driver. So it must share the same ground connection as the microcontroller.

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  31. Hi i checked the circuit in fig.4 but in order to switch the mosfet fast " 60nS "like irlr2905 we need a high current going through the bjt. with R23 1K and supply of 12V we have 0.012A going to the bjt pair. does the 0.012A open the irlr2905 fast ???

    We simulated with r23 160Ohm for 0.075A collector current and R28 R29 500Ohms and on sim it get 50nS to for the Voltage across irlr2905 to go from 24 to 0V, but it takes 500ns for the Vgs to go from 200mV to 12V or 300ns to go from 200mV to 10V.

    Are the values i selected for R23, R28 and R29 correct???

    ReplyDelete
    Replies
    1. Do you want the turn on and turn off times as close to 60ns as possible? Which BJT are you using? Is it a standard BJT or a darlington pair? After knowing the gain (hfe) of the BJT, some simple calculations will give the required values.

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  32. Hi Tahmid
    Could you please let me know if an optocoupler is required between the Microcontroller (Arduino Due) and IR2110 Mosfet Driver. Thanks in advance.

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  33. Hello Tahmid thank u for ur help here . I have a question could I configure fig4 with a boost cap and use it as a high side driver will it work as how it works for low side . I await ur reply

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  34. Can I use Fig4 driver for driving high side N channel mosfet with V_drive = ~20v and V_load = 12v ?
    thanks

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  35. Hi Tahmid
    In fig 5,You say VGS =Vin-0.7v,then if +V-Drive=12v and Vin=5v.Surely 5-0.7(Base,emmittter of Q1)=4.3v turns Q1 on?Then 12v goes through to Gate of Q2 and turns the Fet on?
    I can't figure out why in the top four figures if Vin>0.7v it switches +vDrive through to the gate of the fets but in fig5 it does'nt
    Regards Robin

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  36. hi tahmid
    i am working on two switch buck-bost converter. i am using irfp460 power mosfet.This converter requires the use of two active switches and is designed by combining a buck converter and
    boost converter design in the same topology. Due to this design this converter can work as Buck-only,
    Boost-only or Buck-Boost converter. The input voltage source is connected in parallel with diode D1,
    MOSFET Sboost, load capacitor, C. MOSFET Sbuck is connected between the input
    voltage source and diode D1. The inductor is connected between D1 and Sboost, while D2 is connected
    between Sboost and the output or load capacitor.
    please tell me which side of gate driver i use for sboost and sbuck. also tell me about optocoupler hcpl-3120.

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  37. Hi Tahmid,

    Nice to see this blog. I am facing a problem using ICL7667. This ic is similar in pin configuration like fig 7. With the input B grounded and nothing connected at the output the IC is heating up too much. I have burned one IC. What could be the possible cause.

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  38. Hi tahmid,

    i need your support. what is the value of resistor R9 (10R) in the Fig 2. i placed 10K resistor for R9. I have connected a load (Pneumatic actuator) which operates in 12V (211mA max). but the actuator is not opening. the voltage measured at gate of MOSFET is 11.5V fro the collector of 2nd emitter follower transistor in circuit diagram.

    ReplyDelete
  39. Thanks Tahmid,
    Your blog is very informative & helpful.
    Can we get same performance using discrete components driver as that of special driver ICs? These ICs are costly.

    ReplyDelete
  40. how we can determine the resistance from frequency and capacitor

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  41. how we can determine the resistance from frequency and capacitor

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  42. The Information which you provided is very much useful for SEO Training Learners.. Thank You for Sharing Valuable Information.i like this blog and this is very informative Internet Marketing Company Bangalore | Digital Marketing Company Bangalore

    ReplyDelete
  43. Hello Tahmid,could you please let me know How I can increase the low voltage from the PIC to 15V so that I can control my IGBT (IRG4PC50FD),it is writen in the datasheet that it requires 15V as VGE and I want to control the full bride three phase system driven with SPWM from PIC16F1936,I have the IR2110 and TC427 Drivers only could you please advise on how I can proceed for controlling my AC three phase induction machine?

    Please help.Thank you!!

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  44. very well written, thank you!

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  45. hi bro. Can you plz refer me a high frequency switching mosfet which able to detect approximately 20Mhz output...

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  46. hi tahmid, could you explain me how the totem pole works in fig 5

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  47. Thanks so much for sharing this awesome info! I am looking forward to see more posts by you!
    I need driver

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  48. Hi Tahmid, very illustrative examples, in the fig. 4; how can I do if I have a bipolar power source (+) & (-) in the hv side? should I connect the (-) to common earth in order to have a same circuit reference?

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  49. hello.... can u please tell me how to drive 6 MOSFETs (IRF 3205 or MTP 75N06)? 3 MOSFETS ON PIN NO.14 AND OTHER THREE ON PIN NO. 11 OF IC SG3525A.

    ReplyDelete