How to create PWM with dead time on LTSpice?

Question

I want to create complementary PWM with a dead time to drive 2 MOSFETs at a time. But I think in LTSpice deadtime is something difficult.

Answer 1

You can implement a deadtime quite simply actually by associating a few gates together with two \$RC\$ networks as shown below:

The wanted deadtime is a delay parameter and the two resistors and can be conveniently parametrized. Here, we have 150 ns as shown in the resulting waveforms. It is assumed that the gates deliver 5 V and toggle at a 2.5-V transition level without significant hysteresis.

In an integrated circuit, these dead-times are quite common and designers generate them with a delay brought by several cascaded logic gates as in the below circuit. In the example, the buffer provides the accumulated delay which is also easily adjusted. If LTspice provides these digital gates, it is easy to build:

And there you go with the LTspice version:

Once the digital symbols for the buffer and AND gates are placed on the schematic, right-click on the symbol and add the parameters statements in the SpiceLine to set the high level to 5 V, low to 0 V and the toggling level at 2.5 V - which is, by default \$\frac{V_H+V_L}{2}\$ so ref can probably be ignored.

The cool thing here is that you can add this circuit to more complicated switching converters like those built around half- or full-bridge and add dead-time to any PWM outputs.

Answer 2

You can use the PULSE waveform by adjusting the delay parameter and on time.

The PULSE waveform has these parameters: Vinitial, Von, Tdelay, Trise, Tfall, Ton and Tperiod. Both the high and low-side signals will have the same values for these parameters: Vinitial, Von, Tperiod, Trise and Tfall.

For example, for a 5V 100 kHz pulse train we could have:

Vinitial = 0, Von = 5, Tperiod = 100u, Trise = 1u, Tfall = 1u

and the difference in the high- and low-side signals will only be in Tdelay and Ton.

Let's say we want a dead time of 5u before and after each edge. Then each signal will be on for only 40u instead of 50u.

So the signal which initially starts at 0V can be specified with:

Tdelay = 55u, Ton = 40u

and the one that initially starts high with:

Tdelay = 5u, Ton = 40u

Here is what the signals look like. The traces are:

green - unmodified pulse, blue - signal that starts high, red - signal that starts low

In the general case where you have a pulse which is high for \$\tau_H\$ and low for \$\tau_L\$ (without dead time) and you want a dead time of \$\tau_D\$ between transitions, use the following settings for Tdelay and Ton:

Signal 1: Tperiod = \$\tau_H+\tau_L\$, Tdelay = \$\tau_D\$, Ton = \$\tau_H-2\tau_D\$

Signal 2: Tperiod = \$\tau_H+\tau_L\$, Tdelay = \$\tau_H-2\tau_D\$, Ton = \$\tau_L-2\tau_D\$