User`s guide

Hardware Design

2.4.1.3 OUTPUT CAPACITOR

The output capacitor ripple current is very low due to the continuous inductor currents

with phase shifting. The relationship for capacitor rms current is given by

Equation 2-34.

EQUATION 2-34:

Therefore, the total capacitor ripple current is 1.0 Arms. The output capacitor is made

up of three Rubycon 6.3ZL1500M10X20 1500 μF, 6.3V electrolytic capacitors plus

three 10 μF, 16V multi-layer ceramics in parallel. The electrolytic capacitors are each

rated to 1.82 Arms at 105ºC, and can easily handle the ripple current on their own.

2.5 AUXILIARY POWER SUPPLY

The supply voltages for the primary and secondary side dsPIC DSC device and gate

drive circuitry are generated by a simple low-power flyback SMPS. Figure 2-13

illustrates the main components in the design. The integrated high-voltage proprietary

controlled switch feeds a high-frequency transformer, and multiple secondary windings

tapped-off via a diode/capacitor circuit. The SMPS operates in discontinuous flyback

mode, so that energy is stored in the magnetizing inductance of the transformer during

the switch on-period, and transferred to the secondary circuits during the off-period. A

snubber arrangement is required across the transformer primary to dissipate the

transformer parasitic leakage energy and ensure that the switch voltage does not

exceed its maximum rating. The typical flyback MOSFET waveforms are shown in

Figure 2-14.

FIGURE 2-13: AUXILIARY FLYBACK SMPS

312

cap

Snubber/

Clamp

Control

SEC_17V

SEC_7V

SEC_0V

PRI_13V

PRI_7V