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Z-SERIES

Circuit Description

The Z-Series converters utilize a single ended forward

topology with resonant reset. The nominal 250kHz switch-

ing frequency has been selected to optimize magnetic

element size and switching loss. Electrical isolation be-

tween the primary and secondary sides of the converter

is assured through exclusive use of magnetic coupling for

all signals crossing the primary/secondary barrier. Volt-

age feed-forward is utilized to provide high line rejection.

An under-voltage lockout circuit prohibits the converter

from operating when the line voltage is too low to maintain

the output voltage. The converter will not start until the line

voltage rises to approximately 16.5 volts and will shut down

when the input voltage drops below 15.5 volts. This hys-

teresis voltage reduces the possibility of line noise inter-

fering with the converter’s start-up and shut down. An

input overvoltage circuit is also in place that will shut down

the converter if the input voltage exceeds approximately

60 V, thereby precluding the possibility of exceeding the

voltage rating of the synchronous rectifiers.

A voltage of 0.8V or less will cause the converter to shut-

down. The pin may be left open for normal operation and

has a nominal open circuit voltage of 4.0V.

To achieve the high efficiencies characterizing these con-

verters, synchronous rectifiers have been used in place

of rectifying diodes thus minimizing the conduction losses

associated with those elements.

An internal EMI filter reduces the conducted emissions on

the input power leads. A two-stage output filter reduces

the typical output fundamental ripple to less than 20mV

peak-to-peak.

Output current is limited under any load fault condition to

approximately 125% of rated. An overload condition causes

the converter output to behave like a constant current

source with the output voltage dropping below nominal.

The converter will resume normal operation when the load

current is reduced below the current limit point. This pro-

tects the converter from both overload and short circuit

conditions. The current limit point exhibits a slightly nega-

tive temperature coefficient to reduce the possibility of

thermal runaway.

A synchronization input is provided allowing operation of

the converter synchronously with a user provided fre-

quency source. This input permits synchronization of the

converter to any compatible external frequency source

operating in the band of 225 to 300 KHz and is edge

triggered with synchronization initiated on the negative

transition. This signal should be a negative going pulse

referenced to the input return and have a 20% to 80%

duty cycle. Compatibility requires the negative transition

time to be less than 100 ns with minimum pulse amplitude

of +4.25 volts referred to the input return. In the event of

failure of an external synchronization source, the con-

verter will revert to its own internally set frequency. When

external synchronization is not desired, the sync input

may be left open (unconnected) permitting the converter

to operate at its’ own internally set frequency.

An output adjust pin is provided permitting the user to

adjust the output voltage by approximately ± 5%. Adjust-

ment is accomplished by connecting a resistor brtween

the adjust pin (Pin 10) and +Sense (Pin 9) or –Sense (Pin

11). The direction of the output change is opposite the

sign of the sense pin to which it is connected, that is

connecting to the – Sense pin causes an increase and

connecting to the +sense pin causes a decrease in output

voltage.

Remote sense is provided to compensate for conduction

losses in the connections between the converter outputs

and the load. The use of this sense feature permits com-

pensating for as much as 250 mV in both the output and

return lines. (500 mV each leg, round trip)

An external inhibit port is provided to control converter

operation. The nominal threshold relative to the input re-

turn (pin 2) is 1.4V. If 2.0 volts or greater are applied to the

Inhibit pin (pin 3) then the converter will operate normally.

Design Methodology

The Z-Series was developed using a proven conserva-

tive design methodology that includes selecting radiation

tolerant and established reliability components and fully

derating to the requirements of GSFC PPL-21, MIL-STD-

975 and MIL-STD-1547. Heavy derating of the radiation-

hardened power MOSFET virtually eliminates the possi-

bility of SEGR and SEB. A magnetic feedback circuit is

utilized instead of opto-couplers to minimize temperature,

radiation and aging sensitivity. PSPICE was used exten-

sively to predict and optimize circuit performance for both

beginning and end-of-life. Thorough design analyses in-

clude Worst Case, Stress, Thermal and Reliability (MTBF)

.