Good call, yassin. The BCD part looks pretty nice. The PFM mode is helpful because the pulses are all the same size - it makes your bench testing easier (the EMI testing is a bit more lengthy, but not bad). With PFM the pulse (i.e. peak current) doesn't change depending on load so it's easy to make sure your voltage overshoot and snubbing are right.
Sure, glad to. In PWM, the frequency is fixed and the pulse width varies from 0% to 100% (sometimes the limits are not 0 and not 100 but pretend they are for now). Assume we have a 20kHz PWM frequency. The period of a 20 kHz waveform is the reciprocal of 20kHz = 50us. The pulse width varies from 0 us to 50 us. If the pulse width is 5us, that means the pulse width is on 5us out of 50us, or 10%. If the input voltage is say 75V, then the *average* output voltage is the 10% times the input voltage of 75V = 7.5V. If your circuit needed 15V average on the output, then it would adjust the width of the pulses to 10us or 20% because 20% of 75V = 15V. If the input voltage dropped to 60V, your circuit would automatically change the pulse width to 12.5us because 12.5us is 25% of 50us and 25% of 60V is 15V.
In PFM, we make all pulses 50us wide. (the maximum frequency would be the reciprocal of the period of the single pulse: 1/(50us) = 20kHz). We if we want 10% duty cycle, we put out the 50us pulse and then wait 450us and the fire another pulse (frequency = 1/(50us + 450us) = 2kHz). The average value is still 10% (50us pulse and 450us no pulse). if we need a 16% duty cycle, the wait time would be 263us (50/(50+263) = 20%) and the frequency would be 3.2kHz. The advantage of PFM is that each (50us) pulse is identical - nothing changes between pulses; in PWM the size and energy of the pulse changes so you need to test it very carefully to make sure that the possible pulse widths all look right on an oscilloscope. The second main advantage of PFM is that (almost always) the energy in each pulse is completely transferred to the load.
In contrast, in what is called Continuous Current Modulation, the storage element (the inductor or transformer) doesn't transfer all of its energy every cycle (current stays flowing in the inductor "continuously" - hence the name). When the power devices turn on, they not only have to switch the voltage, they also need to switch the inductor current too. Switching current and voltage means power is dissipated in the semiconductors and you have to check that very carefully on the oscilloscope. The advantage of CCM is that the inductors are smaller and dissipate less power than "Discontinuous Current Mode" operation.
Some circuit topologies, Flyback in particular, are natural candidates for PFM because they MUST transfer all the energy in each pulse to the output or very bad things will happen. The penalty for PFM is that you need to make sure the rest of your circuit does not respond to ANY frequency from 1kHz (5% duty cycle) to 20kHz(100% duty cycle) since your PFM circuit can produce any frequency in that range; with PWM you only need to check the rest of your circuit at the PWM frequency. (Conservation of Misery applies).
Hope this helps.
Very good explanation, i understand the difference now,Sure, glad to. In PWM, the frequency is fixed and the pulse width varies from 0% to 100% (sometimes the limits are not 0 and not 100 but pretend they are for now). Assume we have a 20kHz PWM frequency. The period of a 20 kHz waveform is the reciprocal of 20kHz = 50us. The pulse width varies from 0 us to 50 us. If the pulse width is 5us, that means the pulse width is on 5us out of 50us, or 10%. If the input voltage is say 75V, then the *average* output voltage is the 10% times the input voltage of 75V = 7.5V. If your circuit needed 15V average on the output, then it would adjust the width of the pulses to 10us or 20% because 20% of 75V = 15V. If the input voltage dropped to 60V, your circuit would automatically change the pulse width to 12.5us because 12.5us is 25% of 50us and 25% of 60V is 15V.
In PFM, we make all pulses 50us wide. (the maximum frequency would be the reciprocal of the period of the single pulse: 1/(50us) = 20kHz). We if we want 10% duty cycle, we put out the 50us pulse and then wait 450us and the fire another pulse (frequency = 1/(50us + 450us) = 2kHz). The average value is still 10% (50us pulse and 450us no pulse). if we need a 16% duty cycle, the wait time would be 263us (50/(50+263) = 20%) and the frequency would be 3.2kHz. The advantage of PFM is that each (50us) pulse is identical - nothing changes between pulses; in PWM the size and energy of the pulse changes so you need to test it very carefully to make sure that the possible pulse widths all look right on an oscilloscope. The second main advantage of PFM is that (almost always) the energy in each pulse is completely transferred to the load.
In contrast, in what is called Continuous Current Modulation, the storage element (the inductor or transformer) doesn't transfer all of its energy every cycle (current stays flowing in the inductor "continuously" - hence the name). When the power devices turn on, they not only have to switch the voltage, they also need to switch the inductor current too. Switching current and voltage means power is dissipated in the semiconductors and you have to check that very carefully on the oscilloscope. The advantage of CCM is that the inductors are smaller and dissipate less power than "Discontinuous Current Mode" operation.
Some circuit topologies, Flyback in particular, are natural candidates for PFM because they MUST transfer all the energy in each pulse to the output or very bad things will happen. The penalty for PFM is that you need to make sure the rest of your circuit does not respond to ANY frequency from 1kHz (5% duty cycle) to 20kHz(100% duty cycle) since your PFM circuit can produce any frequency in that range; with PWM you only need to check the rest of your circuit at the PWM frequency. (Conservation of Misery applies).
Hope this helps.
i just found another cheaper one, **broken link removed**
it is PFM ( pulse frequency modulation)
and few external component
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