Phase lead: the delay chain and the ZVS floor

A driver that fires on the current zero is already late. By the time the comparator has decided, the logic has propagated, the gate driver has swung and the switch has actually opened, the current it was supposed to break has been through zero and out the other side for two hundred nanoseconds. The lead is how you pay that back.

What the lead is made of

Two things, and only one of them is fixed.

The first is the delay chain: the comparator, the logic, the driver, the output stage, the gate transformer and the switch itself. On the bridge these notes were written around it adds up to about 184 ns, of which 115 is the switch alone. It is a datasheet sum rather than a measurement, and every link in it carries a caveat.

The second is the ZVS floor, and it is not a number anybody chooses. It is the amount of residual current the bridge needs in order to recharge its own node during the dead time, and it comes out of a charge balance:

Q(t_dead) = I_pk/w · [cos(w(dt - t_dead)) - cos(w·dt)] >= C_node · V_bus

Solve that for the smallest dt and you have the floor. On the reference bridge at 490 kHz it is 80 ns.

Why turning off exactly at zero is the failure case

It reads like the ideal and it is the opposite. Open the switch precisely at the current zero and there is no current left, so nothing recharges the node, so the opposite switch turns on into the full bus. At 440 V that transition costs roughly 440 µJ per device, which at half a megahertz is kilowatts of heat that did not need to exist.

ZCS and ZVS trade against each other. Leftover current is the price of soft switching, and the lead is how you buy it.

What this means at the bench

1. Wind the inductor the calculator gives you, less whatever the layout already contributes.
2. Power up and put a probe on the bridge output.
3. Look for a small bump at the end of the edge. That is the optimum, not a defect.
4. Turn from small to large. Falling short is the more dangerous side.
5. Repeat it warm. A hot bridge asks for more, and the setting moves.

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On a ramped coil the frequency this is all computed at does not stay put, and a network holding a constant angle then delivers a changing number of nanoseconds. Worked across a real ramp on the frequency page.

The phase lead calculator does the arithmetic and prints the zone rather than a single number, because a single number would promise a precision the inputs do not have.