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From: "Harry Dellamano"
Subject: The Care and feeding of ZVS Power Supplies
X-Newsreader: Microsoft Outlook Express 6.00.2720.3000
Date: Sat, 28 Dec 2002 21:25:34 GMT
NNTP-Posting-Date: Sat, 28 Dec 2002 16:25:34 EST
This is in answer to another thread going on about Zero Voltage Switching
Seems to be a lot of confusion in the control of the main switch (FET's)
off time (Toff). In their proposed control loop they are controlling the
peak charging current by the output voltage to determine the switch's on
time (Ton). Toff can be constant or it can be inversely proportional to the
input voltage (voltage feedfoward), output voltage, (great for charging
caps), the difference between input and output voltages or and combination
to get the desired effect. The Toff time can also be terminated by zero
volts in any inductor or any voltage crossing a threshold as in drain
voltage crossing zero volts. My favorite way is to detect the negative gate
spike caused by the drain voltage dropping to zero. At about 15V the FET's
drain-gate capacity has an abrupt change, sucking charge in the gate. The
Toff can also be limited by voltage clamps and such. Ton can also be
controlled in many different ways.
Notice that this forces the switching frequency to be variable. In some
instances it will go to zero so it must be kick started.
Take a spreadsheet and some simple algebra to calculate the different
combinations of above to determine Ton, Toff, frequency, peak and average
currents for all line and load changes. In not all instances will ZVS
benefit your design but maybe 25% of the time it is the way to go.
I'm sure analog and many others can add much to this discussion.
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