Using the TLP250 Isolated MOSFET Driver - Explanation and Example Circuits

I’ve already shown how to drive an N-channel MOSFET (or even an IGBT) in both high-side and low-side configurations in a multitude of ways. I’ve also explained the principles of driving the MOSFETs in these configurations. The dedicated drivers I’ve shown so far are the TC427 and IR2110. Some people have requested me to write up on MOSFET drive using the very popular TLP250. And I’ll explain that here.

The TLP250, like any driver, has an input stage, an output stage and a power supply connection. What’s special about the TLP250 is that the TLP250 is an optically isolated driver, meaning that the input and output are “optically isolated”. The isolation is optical – the input stage is an LED and the receiving output stage is light sensitive (think “photodetector”).

Before delving any further, let’s look at the pin configuration and the truth table.

Fig. 1 - TLP250 Pin Configuration

Fig. 2 - TLP250 Truth Table

Fig. 1 clearly shows the input LED side and the receiving photodetector as well as the totem-pole driver stage. Pins 1 and 4 are not internally connected to anything, and hence are labeled N.C. meaning no connection.

Pin 8 is VCC – the positive supply. Pin 5 is GND – the ground supply or the return path for the driving power supply. The supply voltage must be at least 10V. The maximum voltage is dependent on the operating temperature. If the temperature is lower than 70°C, up to 30V can be used. For temperatures between 70°C and 85°C, up to 20V can be used. However, there shouldn’t be a need to use higher than 20V anyways. In most cases, you’ll be using 12V or 15V or perhaps in some cases 18V.

Pins 2 and 3 are the inputs to the LED, anode and cathode respectively. Like regular LEDs, it has an input forward voltage and a peak forward current. The forward voltage will typically be between 1.6V and 1.8V. The forward current should be less than 20mA. The threshold input current for output transition from low to high is typically 1.2mA, but may be as high as 5mA. Thus, 10mA current should be good.

Even though pins 6 and 7 are shown to be internally connected, the output should be taken from pin 6 as the image - datasheet - shows pin 6 labeled as Vo (Output). Output voltage will tend to rise to supply voltage when high (it will actually be slightly lower) and fall to ground level when low.

The TLP250, being an optically isolated driver, has relatively slow propagation delays (not to say that optically isolated drivers can’t be fast; there are optically isolated drivers faster than TLP250). The propagation delay time will typically lie between 0.15µs and 0.5µs. An important thing to remember is that the datasheet specifies the maximum operating frequency to be 25kHz. I’ve used the TLP250 for frequencies up to about 16kHz.

That covers the different parameters related to TLP250. Now let’s go to the design stage and look at a few circuits. One thing you MUST remember to do when designing circuits with TLP250 is that, a 0.1µF bypass capacitor (ceramic capacitor) should be connected between V+ (pin 8) and V- (pin 5). This capacitor stabilizes the operation of the high gain linear amplifier in the TLP250. Failure to provide this capacitor may impair the switching property. The capacitor should be placed as close to the TLP250 as possible. The closer, the better.

Fig. 3 - Non-Inverting Isolated Low-Side MOSFET Driver

Fig. 3 shows a typical circuit for using the TLP250 as a MOSFET driver. VIN is the input drive signal that dictates the output state. Remember that VIN is referenced to Signal Ground. And that the TLP250 ground and load ground are referenced to the power ground, ie Vsupply and VMOS share the same reference ground as can clearly be seen from the circuit diagram and this ground is separate from Signal Ground. This clearly illustrates the isolation in MOSFET drive as the driving signal is isolated from the load supply.

When VIN = 1, Q1 is driven from the supply voltage (Vsupply) – the gate is pulled up to Vsupply level. Q1 turns on and current flows through the load – the load is driven from VMOS via the MOSFET.

When VIN = 0, Q1 is driven low – the gate is pulled down to its source level. Q1 turns off and the load is off.

Vsupply could be between 10V and 15V – 12V being a very common level used. R1 should be calculated by you depending on the amplitude of the input signal. I’ll give an example to clearly show you how (if you don’t know that already).

I’ve said above that 10mA (= 0.01A) for the forward current for the LED is a good value to use. So I’ll take that. Let’s say that the TLP250 is being driven from a microcontroller and the amplitude for the signal is 5V. I’ve said above that the forward voltage for the LED would typically be between 1.6V and 1.8V – I’ll take it to be 1.8V for this example.

So, V = (5.0 – 1.8)V = 3.2V

V = IR

R = V/I = 3.2V/(0.01A) = 320Ω

R2 is the gate resistor. If you’re curious about why I used R3, read here:

http://tahmidmc.blogspot.com/2012/10/magic-of-knowledge.html

C1 is the decoupling capacitor I talked about above. This MUST always be used and MUST not be omitted. I’ve added C2 for filtering/smoothing, as a bulk capacitor.

Let’s look at a few more circuits:

Fig. 4 - Inverting Isolated Low-Side MOSFET Driver

This circuit in Fig. 4 is similar to the above circuit in Fig. 3, with the difference being that the circuit in Fig. 3 shows a non-inverting driver (VIN = 1 drives the MOSFET on and VIN = 0 drives the MOSFET off) whereas Fig. 4 shows an inverting driver (VIN = 0 drives the MOSFET on and VIN = 1 drives the MOSFET off). How this has been configured to be an inverting driver is extremely simple to understand – the LED now turns on when VIN = 0 and turns off when VIN = 1. Like Fig. 3, Fig. 4 also shows an isolated driver: +VS is isolated from Vsupply and VMOS.

 Fig. 5 - Non-Inverting Non-Isolated Low-Side MOSFET Driver

Fig. 5 shows a non-inverting non-isolated driver. By shorting Signal Ground and Power Ground, isolation has been gotten rid of. Vsupply and VMOS share the same ground as the signal ground to which VIN is referenced.

 Fig. 6 - Non-Inverting Isolated High-Side MOSFET Driver

Fig. 6 shows the TLP250 being used as a high-side driver. Here in this circuit, there are 3 “grounds” – that of the signal ground to which VIN is referenced, that of Vsupply and that of VMOS.

When VIN = 1, Q1 gate is pulled up to the level of Vsupply (with respect to source). Since this is above the level of the source (which is connected to Vsupply return/ground), the MOSFET turns on and there is a current from VMOS through Q1 through the load, turning the load on.

When VIN = 0, Q1 gate is pulled down to the level of source and Q1 is turned off. There is no current through the load and the load is off.

By having the MOSFET source share the same ground as the TLP250 drive section and keeping this ground separate from the VMOS ground, Vsupply is easily used by the TLP250 to drive the MOSFET operating as a high-side switch.

And that’s it. The TLP250 is a useful little chip, making isolated MOSFET drive extremely simple. One last note is that while I've shown the circuits for MOSFET drive, they can easily be used (with the same circuit) for IGBT drive (of course, you replace the MOSFET with the IGBT).

I hope that my explanation of the application of the TLP250 and the circuit examples I provided help you in designing your own circuits using the TLP250 for optically isolated MOSFET (or IGBT) drive. Feel free to post your comments, feedback and suggestions!