AdvancedA-004-002-010
Excessive rectifier peak current and abnormally high peak inverse voltages can be caused in a power supply by the filter forming a:
B
Answer
Measurements and troubleshooting
Type
A
parallel resonant circuit with the first choke and second capacitor
B
series resonant circuit with the first choke and first capacitor
C
short circuit across the bleeder
D
tuned inductance in the filter choke
Answer Notes
A choke input filter consists of a choke (inductor) in series followed by a capacitor in parallel with the load. From the perspective of the rectifier, this combination forms a series LC circuit to ground.
If the first choke and the first capacitor are inadvertently chosen such that they form a series resonant circuit at the ripple frequency, the impedance of the filter drops drastically. This near-zero impedance allows excessive peak currents to flow through the rectifier, which can cause abnormally high peak inverse voltages across the diodes.
This is why power supply filters must be designed with LC values that resonate well below the lowest expected ripple frequency, preventing catastrophic series resonance.
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In the design of a power supply, the designer must be careful of resonance effects because the ripple voltage could build up to a high value. The components that must be carefully selected are:
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In a properly designed choke input filter power supply, the no-load voltage across the filter capacitor will be about nine-tenths of the AC RMS voltage yet it is advisable to use capacitors rated at the peak transformer voltage. Why is this large safety margin suggested?