7/9

AN829 APPLICATION NOTE

Then the maximum rms voltage across the sense is:

RMS sense voltage = 15m

Ω 16A = .24V

Then R8 = R9 = .24V / 60

µA # 4kΩ

Critical current amplifier gain occurs when the current error amplifier slope exceeds the oscillator slope. This

condition occurs when:

IMPLEMENTATION AND SWITCHING BEHAVIOR

Using a double-sided PCB significantly reduces the parasitic inductances of the circuit:

– parasitic inductance Lp1 and Lp2 shown in figure 1 are intrinsic characteristics of the ISOTOP module

itself; these values are very low (less than 10nH).

– parasitic inductances Lp3 and Lp4 are due to the ISOTOP/capacitor loop. The proposed layout re-

sults in about 20 nH for Lp3+Lp4.

These values mean that the total voltage overshoot during the turn off of the MOSFET is limited to about 30V

with a di/dt of 1000A/sec with no snubber.

– parasitic inductance Lp5 is a few nH due to the use of an active current sense in a TO- 218 package.

This results in an excellent signal/ noise ratio at the current error amplifier input. The following mea-

surements have been made with the iron powder inductor as described in paragraph 4.

Figure 5 shows the most important signals with an input voltage of 208Vac and an output power of 1600W. To

show the current ripple more clearly, one second persistence has been used. The slight overshoot of the current

error amplifier output during the mains zero voltage crossing is due to the rise of the inductor permeability at low

induction. Indeed, the inductor size optimisation requires operation with 50% saturation of the iron at maximum

peak current.

Figure 6 shows average values of the waveforms in figure 5.

Figure 7 shows the input current and voltage with 120Vac mains and 1600W output Power.

Figure 8 shows the drain voltage and source current during turn off. Note that the source current probing cre-

ates a parasitic inductance, limiting the di/dt. Thus there is no significant turn off overvoltage.

Figure 9 shows the diode recovery current with a forward current of 20A and a di/dt of 700A/

µsec. Note that the

to the low parasitic inductance of the gate drive. Thermal measurements have been performed enabling the