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:
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!
