Some Advice on a Failed Totem-Pole Circuit

Question

I'm adapting a pre-existing circuit based on an ATTiny13. The original circuit has PWM output pins from the ATTiny either sourcing or sinking current for the centre point of two standard LEDs in series, and I wanted to replicate it with much higher power LEDs that need more current than can safely be supplied by the ATTiny. I therefore tried to create a totem-pole arrangement with a PMOS and an NMOS to extend the current supply capability of the pin:

However, something went wrong, and my safety resistor released the magic smoke. Using a thermal camera, I could see that the mosfets were also hot. After doing some reading around, my only idea is that the PMOS and NMOS were both being activated at the same time, leading to a short. The PMOS is a SOT23 3401 (A19T) and the NMOS is a 3402 (A29T). The Vgs(th) is -1V and 1V respectively, and the circuit uses 5V from USB (or 3.7V from a lipo). Assuming that I'm right about both mosfets being active while the switching voltage moves between 4V and 1V, what should I think about to create a version of this circuit that actually works (as in, only one mosfet is ever on)?

Should I search for mosfets with a Vgs(th) larger than 2.5V? Should I swap the PMOS and NMOS positions, or use NMOS for both positions? Ideally there would be as little voltage drop as possible. I could also replace the mosfets with a PNP and NPN BJTs.

Any prompts or advice would be greatly appreciated.

Answer 1

what should I think about to create a version of this circuit that actually works?

You need a really fast transition on the gate driver, if it isn't fast enough both fets will be on at the same time. Fets in a stage like this may also need to be matched. Other things that can kill the setup are ESD and too much current through either device, and over voltage.

Simulating the stage in spice is recommended.

If the load is inductive, use a flyback diode If you are handling the circuit, or there are wires attached, make sure you have proper grounding and you may also consider TVS diode to shunt ESD to ground. You may want to consider a small current limiting resistor if you don't need the full current of the rail.

Answer 2

As others stated, cross-conduction is occurring. How much current flows depends on the gate voltage required for device turn-on.

To demonstrate, a half-bridge configuration is simulated using some random transistors in LTspice with fairly low turn-on gate voltage is shown below. The X-axis is the input voltage, V(in). The Y-axis is the current through the transistors.

In this case, the cross-conduction current is a bit over 3A and occurs with V(in) in the 2.0 to 2.9 V range. Your circuit could have a larger or smaller cross-conduction current. To prevent this you can use driver ICs that prevent cross-conduction or drive the gates independently so cross-conduction is avoided.

When designing H-bridge amplifiers we drive each gate independently to avoid cross conduction issues. Experimental firmware is created so dead-time can be adjusted on the fly to determine an appropriate dead-time.