Sunday, March 8, 2015

Circuit Schematic of the 12V to 5V USB Converter using SCR

Eltronicschool. - Here is Circuit Schematic of the 12V to 5V USB Converter using SCR. SCRs are the switching devices for this unusual DC to DC converter that is suited for the ubiquitous 5V USB power source as well as numerous other applications. This novel power circuit topology uses an intermediate resonant link that facilitates self-commutation for both SCRs – while SCRs are easy to turn on, they tend to be difficult to turn off in DC applications.

Circuit Schematic


Circuit schematic like in figure 1 above is Circuit Schematic of the 12V to 5V USB Converter using SCR. This circuit schematic can works  as follow :

For firing SCR1, the minimum delay time is a function of the charging of C8 –this delay is extended by the effect of the voltage regulator. D3 is the TI TL431 adjustable shunt voltage reference that is strapped for minimum (2.5V). Q7 compares this reference voltage to the feedback voltage through the feedback voltage divider that consists of R16 & 17. Q6 both inverts the feedback signal and references it to common potential so that it may shunt C8. Q5 turns on when there is voltage across the capacitor network –it essentially tells the firing circuit that the capacitor network is empty and needs a new charge. Q5 & 6 also form an AND function so that both conditions must be satisfied before C8 is allowed to charge. When C8 charges to approx 8.6V,Q8 fires and triggers SCR1. SCR1 subsequently charges the capacitor network and does not stop at 12V because at that point the inductor is fully charged and must keep conducting until the capacitor network voltage reaches approx 24V.

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