I’ve already shown how to drive an N-channel MOSFET (or evenan 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 theseconfigurations. The dedicated drivers I’ve shown so far are the TC427 andIR2110. Some people have requested me to write up on MOSFET drive using thevery popular TLP250. And I’ll explain that here.
The TLP250, like any driver, has an input stage, an outputstage and a power supply connection. What’s special about the TLP250 is thatthe TLP250 is an optically isolated driver, meaning that the input and outputare “optically isolated”. The isolation is optical – the input stage is an LEDand the receiving output stage is light sensitive (think “photodetector”).
Before delving any further, let’s look at the pinconfiguration and the truth table.
Fig. 1 - TLP250 Pin Configuration
Fig. 2 - TLP250 Truth Table
Fig. 1 clearly shows the input LED side and thereceiving photodetector as well as the totem-pole driver stage. Pins 1 and 4are not internally connected to anything, and hence are labeled N.C. meaning noconnection.
Pin 8 is VCC – the positive supply. Pin 5 is GND – theground supply or the return path for the driving power supply. The supply voltagemust be at least 10V. The maximum voltage is dependent on the operatingtemperature. If the temperature is lower than 70°C, up to 30V can be used. Fortemperatures 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’llbe using 12V or 15V or perhaps in some cases 18V.
Pins 2 and 3 are the inputs to the LED, anode and cathoderespectively. Like regular LEDs, it has an input forward voltage and a peakforward 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 foroutput transition from low to high is typically 1.2mA, but may be as high as5mA. Thus, 10mA current should be good.
Even though pins 6 and 7 are shown to be internallyconnected, the output should be taken from pin 6 as the image - datasheet - shows pin 6 labeledas 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, hasrelatively slow propagation delays (not to say that optically isolated driverscan’t be fast; there are optically isolated drivers faster than TLP250). Thepropagation delay time will typically lie between 0.15µs and 0.5µs. An important thing to remember is that the datasheet specifies the maximum operatingfrequency 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 MUSTremember to do when designing circuits with TLP250 is that, a 0.1µFbypass capacitor (ceramic capacitor) should be connected between V+(pin 8) and V- (pin 5). This capacitor stabilizes the operation of the highgain linear amplifier in the TLP250. Failure to provide this capacitor mayimpair the switching property. The capacitor should be placed as close to theTLP250 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 aMOSFET driver. VIN is the input drive signal that dictates the output state.Remember that VIN is referenced to Signal Ground. And that the TLP250 groundand load ground are referenced to the power ground, ie Vsupply and VMOS sharethe same reference ground as can clearly be seen from the circuit diagram and this ground is separate from Signal Ground. Thisclearly illustrates the isolation in MOSFET drive as the driving signal isisolated 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 itssource level. Q1 turns off and the load is off.
Vsupply could be between 10V and 15V – 12V being a verycommon level used. R1 should be calculated by you depending on the amplitude ofthe input signal. I’ll give an example to clearly show you how (if you don’tknow that already).
I’ve said above that 10mA (= 0.01A) for the forward currentfor the LED is a good value to use. So I’ll take that. Let’s say that theTLP250 is being driven from a microcontroller and the amplitude for the signalis 5V. I’ve said above that the forward voltage for the LED would typically bebetween 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 MUSTalways 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 differencebeing that the circuit in Fig. 3 shows a non-inverting driver (VIN = 1 drives the MOSFET onand 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 beenconfigured to be an inverting driver is extremely simple to understand – the LEDnow turns on when VIN = 0 and turns off when VIN = 1. Like Fig. 3, Fig. 4 alsoshows 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. Byshorting 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 isreferenced.
Fig. 6 - Non-Inverting Isolated High-Side MOSFET Driver
Fig. 6 shows the TLP250 being used as a high-side driver. Herein this circuit, there are 3 “grounds” – that of the signal ground to which VINis 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 Vsupplyreturn/ground), the MOSFET turns on and there is a current from VMOS through Q1through the load, turning the load on.
When VIN = 0, Q1 gate is pulled down to the level of sourceand 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 theTLP250 drive section and keeping this ground separate from the VMOS ground,Vsupply is easily used by the TLP250 to drive the MOSFET operating as ahigh-side switch.
And that’s it. The TLP250 is a useful little chip, makingisolated 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 indesigning your own circuits using the TLP250 for optically isolated MOSFET (or IGBT) drive.Feel free to post your comments, feedback and suggestions!
