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PART II

5 Circuit description

5.1 DC power supply

Tosuppressnoiseonthesupplywiresalterisbuildaroundthecurrentcompensated

choke L1. Furthermore the RC snubbers across the transformer prevent unwanted oscil-

lations during transition time. The diode D7 in conjunction with R16 and C19 clamps

the transient voltage spikes across the switch transistor Q1.

When the supply is switched on, the capacitor C6 is charged by means of R1 and when

the voltage exceeds approx. 8.7V the con troller U1 turns on. After a few cycles power is

delivered from the bootstrap winding of the transformer to maintain the supply to the

controller U1. When switching of the supply voltage, the power from the transformer

disappears. This means the supply voltage to U1 decreases to approx. 7.6V and the

controller U1 switches off.

Therectiedvoltagefromthebootstrapwindingisalsousedtoregulatetheoutput

voltages, which mainly is determined by R8 and R11. The feedback voltage is led to U1

pin 2. The resistors R10 and R9 and the capacitor C17 form the compensating circuit

for the voltage regulation.

Thecontrollerisworkingataxedfrequencyofapprox.100kHz,determinedbyR7

and C16.

TheswitchcurrentissensedbyR15andledtoU1pin3afterbeinglteredbyR14

and C18. The current sense voltage is a ramp voltage, which is compared to a DC level

determined by the voltage regulation circuit. When the ramp exceeds the DC level, the

controller U1 switch off the FET Q1. This current sense circuit forms a current regulation

loop,whichisstabilizingtheoverallregulationandpreventshighcurrenttodamage

the switch transistor Q1.

When Q1 is on, the output diodes D3 and D4 is reversed biased and current to the load

is delivered from the capacitors C12 and C13. During the off time of Q1, the transformer

outputs deliver the currents to the capacitors C12 and C13 and to the load.

ThechokesL2andL3arepartsoflowpasslterstosuppressvoltagespikes.

Becauseofthe converter principle,which isaringing chokeyback type,without

direct feedback from the output the voltages, the output voltages are load dependent.

Atinputvoltageunderapprox.15Vandatfullload,theconverter”changesfrequency”

and gets an irregular duty-cycle. That is normal behaviour.

At low input voltage the start up time can be as long as 4 seconds, depending on the load.

0627

COBHAM SAILOR H1252B/TT-3608A User Manual

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