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What is the difference between PWM control and VFM control?
DC/DC Switching RegulatorsFunction
Ricoh’s DCDC converters are pulse-width modulation (PWM) controlled or variable-frequency modulation (VFM) controlled. VFM is also called pulse-frequency modulation (PFM).
PWM Control
The oscillator frequency is constant and the pulse width (ON time) changes according to the amount of output load.
VFM Control
The pulse width (ON time) is constant and the oscillator frequency changes according to the amount of output load.
The features of VFM control and PWM control are as described below.
| Oscillator Frequency | Pulse Width (Duty) |
Light Load Condition | Noise Suppression*1 |
||
|---|---|---|---|---|---|
| Efficiency | Ripple Voltage | ||||
| VFM Control | Change | Constant | Good | High | Difficult |
| PWM Control | Constant | Change | Not Good | Low | Easy |
- *1Noise suppression of VFM control can be difficult because oscillator frequency is inconsistent and it affects noise level.
Noise suppression of PWM control is relatively easier because oscillator frequency is constant and it does not affect noise level.
Ricoh’s DCDC converters include the products that are only operated by PWM control and the products that can be automatically switched between PWM mode and VFM mode according to the load condition.
RP506K Efficiency vs. Output Current
Forced PWM Control
The general PWM control DCDC converters may cause ringing in the Lx voltage because there is a zero current period to prevent a reverse coil current in low load condition.
However the forced PWM control DCDC converters do not cause ringing by forcing them into PWM control in low load condition.
Although the reverse coil current is present, the reverse current is not wasted because it flows to the output in the next cycle.
General PWM Control
Forced PWM Control
Ricoh forced PWM control DCDC converters are able to switch automatically between PWM and VFM control.
If one prefers that the supply current and efficiency are most important, please select automatic PWM/VFM switching mode.
In that case the DCDC converter operates in VFM control during low load conditions.
If one prefers that no ringing in low load condition should appear and operation with a constant frequency, please select fixed forced PWM control.
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