General circuit description

STEVAL-ISA005V1

2/5

1

General circuit description

Input capacitor C1 is charged from line via a one way rectifier consisting of diodes D1 and

D2. Two diodes in series are used for EMI reasons to sustain burst pulses of 2kV.

Capacitor C1 together with capacitor C2 and inductor L1 forms an EMI filter. The DC voltage

at C2 is then applied to the VIPer12 that works as a high side switch. It means that the IC

and corresponding supply and feedback loop circuitry is floating. The IC supply circuit

consists of the high voltage diode D4, ceramic capacitor C7, low voltage diode D7 and

capacitor C4. The voltage feedback loop is provided via zener diode D3, resistor R3 and

capacitor C6. The diode D7 between capacitor C7 and C4 ensures proper start-up of the

converter.

Thanks to this diode the feedback loop circuit is separated from the supply circuit. The

VIPer12 internal start-up current source charges the IC supply capacitor C4 to a specified

start-up threshold voltage of about 16V. As soon as the voltage of C4 reaches the start-up

threshold, the internal 60kHz oscillator sets the internal flipflop and the internal highside

Power MOSFET is turned on through the output driver. The Power MOSFET applies the

bulk capacitor C1 and C2 high voltage to the power diode cathode and to one terminal of the

inductor. Since the voltage at the output capacitor C3 connected to the inductor's second

terminal is much lower than the input bulk capacitor voltage, inductor current will ramp-up.

As soon as the inductor current ramp reaches the VIPer's internal set-point defined by the

feedback loop, the internal power switch turns off. The inductor keeps the direction of the

current flowing and it reverses the voltage at C3. The inductor current then flows through the

forward biased D5 diode and charges output capacitor C3.

In this switch-off phase the VIPer12 source terminal sees a negative level of the forward

biased D5 (when referenced to ground) so it can be considered as grounded. This allows

the inductor current to flow also through D4, supply the VIPer12 and give the feedback

information concerning the output voltage. The converter output voltage at the 15V terminal

is determined by the voltage drop across zener diode D3 together with the voltage drop

across the resistor R3 and FB pin voltage. Resistor R3 limits the feedback current to a safe

value lower than the maximum rating specified in the datasheet. Capacitor C6 protects the

FB input against EMI. Regulator U2 accommodated in a DPAK or SO-8 package is optional

and can be assembled if the power supply for a microcontroller or logic part is required. The

DPAK package version of U2 is dedicated for 60mA output current option while the U3 in

SO-8 package provides a maximum output current of 20mA.