# Is this Half-Bridge waveform right?

This started with my question here there is no need to read it, I put it here for additional info.

I'm trying to drive two MOSFETs in Half-Bridge topology and I finally could get the pulse transformer working correctly using a IR2110 working as a two low-side driver. Here is the schematic:

simulate this circuit – Schematic created using CircuitLab

Probe 1 and Probe 2 are my oscilloscopes probes and here are the resulting waveforms when driving the IR2110 through a microcontroller generating a 200KHz Half-Bridge PWM:

There is some ringing but I assume that's because I'm testing in a protoboard and there is a lot of distributed capacitance. My real concern is about the waveform. The transformer in the schematic has only one secondary but my transformer has two secondaries in opposite directions for each MOSFET.

The yellow waveform is Probe 1 as you can see I get the expected waveform with a dead-time at the reference level (the small '1' yellow mark on the left of the screen) but on the secondary side (the blue signal, Probe 2) this dead-time is at about 10V and it should be at reference level otherwise MOSFET is active at 10V.

Am I right or I'm missing something?

• Also, you should wait a day or so before accepting an answer to stimulate discussion - others may not answer now since you've already awarded the reputation bonus. – Adam Lawrence Sep 30 '14 at 20:46
• Thank you Adam! So if I MOSFET gate on D2 cathode I will get what I'm looking for? I'm testing the circuit simulating a MOSFET gate with a 4700pF capacitor (a little big I know) and that is the waveform I get, do I need the PNP anyway? – Andres Sep 30 '14 at 20:50
• Your circuit is AC-coupling the waveform into a 'DC-restore' gate drive circuit. The gate connection should be at the other side of D2 (minus the 100nF cap) and of course you will need a PNP transistor for fast turn-off. – Adam Lawrence Sep 30 '14 at 20:50
• There's no path to discharge the gate other than the resistor, which will take much longer than your total off-time to achieve. You need active pull-down. – Adam Lawrence Sep 30 '14 at 20:52
• The gate will be discharged by the transformer and cap taking gate voltage to -0.7 volts courtesy of the clamping diode. – Andy aka Sep 30 '14 at 22:29

Your circuit is a DC-restore gate drive scheme. The primary waveform gets AC-coupled and DC-restored on the secondary side.

Probe across the 4.7k resistor and you'll see what the MOSFET is seeing. (Also, 100nF is a little excessive for a MOSFET gate - go with 1 to 10nF to keep it real.) When you look at the gate, you should see that this scheme needs an active pull-down as there's no discharge path for the gate capacitance other than the resistor. Commonly a PNP transistor is used for this function.

(You may also need to damp/clamp the coupling capacitors depending on the amount of ringing you get.)

Assuming you only want the positive parts of the pulses, here's another configuration that's suitable for a half-bridge (symmetrical drive for each FET) - notice no coupling capacitors.

• Thank you Adam! With the first circuit, can I use double secondary or I have to use one transformer for each MOSFET? Which circuit would you recommend? – Andres Sep 30 '14 at 21:53
• I measured on the cathode where you suggested changing the capacitor to 1nF and you are right, discharge is very slow and the voltage doesn't even reach 0V is about 5-6V above. Tomorrow will try the first circuit you suggested. – Andres Oct 1 '14 at 1:06
• Ok, I tried your second circuit but it didn't work well with duty cycles bigger than 50% because there is no AC coupling. I ended up using your first circuit, it works great! I will have to use two driver as suggested by @Andy aka – Andres Oct 1 '14 at 16:54

Am I right or I'm missing something?

Yes you are missing something and that something is, I believe, that one transformer feeds only one MOSFET. One transformer should be connected to OUT1 and GND while the other connects to OUT2 and GND - the drives to the MOSFETs need to remain seperate.

• So this (source: fig.3 here) doesn't really work? – Andres Sep 30 '14 at 18:37
• That figure should work fine (conceptually) - obviously its missing some components but driving the FETs like that is valid IMHO. – Adam Lawrence Sep 30 '14 at 20:47
• I'm concerned that driving the fets from one transformer may lead to shoot thru problems. – Andy aka Sep 30 '14 at 22:31