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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Well summarised Marko! As you said in your previous post capacitive turn-on losses dominate in MOSFET inverters running at high-frequency and from high-voltage. For this reason Zero-Voltage-Switching is far more beneficial than Zero-Current-Switching. (Mosfets can already switch large currents in nano-seconds and with no current tailing or latchup. So ZCS really has little benefit to offer, but ZVS can potentially eliminate capacitive turn-on dissipation and also reduce energy dissipated due to forced diode reverse-recovery.)
As an example a bridge leg containing two of the very common IRFP460 devices running at 250kHz has to charge and discharge the parallel combination of both device's output capacitances 500 thousand times ever second! The device datasheet specifies Coss as 870pF so that gives a total capacitance of 1740pF for two devices. On a 340VDC bus that represents 100.5uJ of stored energy. 500 thousand switching transitions per second gives 50.3 watts of total dissipation with no load connected. This is the power wasted just in order to make the mid-point voltage of the bridge leg slew back and forth between 0 and 340VDC. Even when split between the two devices this figure still represents a significant portion of their practical power dissipation! And the total dissipation is doubled for a full-bridge consisting of 2 bridge legs, and tripled for a 3-phase inverter.
What's more, this simple analysis does not take into account the reverse transfer capacitance ("Miller" capacitance) of the MOSFETs which further increases losses. It also doesn't include the losses due to forced reverse-recovery of the fast recovery diodes used to bypass the MOSFETs internal body-drain diodes.
These are the reasons why most modern SMPSU's employ some sort of zero-voltage switching scheme to remain competitive in the market place in terms of efficiency and particularly power density. The phase-shifted ZVT full-bridge converter being one of the most popular.
-Richie,
PS. Jan your basic calculation of the capacitive turn-on losses was right but you forgot that there are two switching transitions per cycle and that both devices output capacitances must be charged and discharged in each switching event! Your figure of 110pF for Coss of an IRFP450 also sounds very low, certainly for any practically mounted device.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I got that figure from International Rectifier datasheet for IRFP450N. The conditions are, VGS = 0V, VDS = 0V to 400V and it does include miller capacitance. However the "static" output capacitance at VDS = 25V is 210pF.
You are right, I forgot both capacitances must be discharged, so the figure grows to 2.8W. This is a figure for single device, each device switches only once every cycle.
But it's quite possible that the diode reverse recovery losses could be bigger than that.
I don't like deadtime because the GDT absolutely must not ring, as even a few volt spike could turn the other device on in the beginning of every deadtime (when both outputs are meant to be at 0V).
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> You are right, I forgot both capacitances must be discharged, so the figure grows to 2.8W.
Dont forget that the device capacitances must both be charged and discharged. Both of these processes incur losses...
When the top MOSFET turns on it must discharge it's own Cds capacitance into its channel, whilst at the same time supporting the current required to charge the lower device's Cds to the bus voltage.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
GeordieBoy wrote ... Dont forget that the device capacitances must both be charged and discharged. Both of these processes incur losses... When the top MOSFET turns on it must discharge it's own Cds capacitance into its channel, whilst at the same time supporting the current required to charge the lower device's Cds to the bus voltage.
I meant for single device. For both it is 5.6W. Each device only turns on once in a cycle, on turn off it doesn't waste any capacitive power (because the other one does ).
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Bridge cross-conduction
This is a figure for single device, each device switches only once every cycle.