I wanted to share some insights I gained by spicing up a normal supply, in this case a 12V/4Adc, consisting of a transformer, bridge rectifier and smoothing capacitor. In another thread some of us gave their opinions about the choice of the required components, and I tried to verify.
- The transformer can be modeled with an ideal ac-source plus interiour resistance, as we know from any DC-supply. Almost all losses are due to resistance of the windings. The manufacturer specifies a certain obtainable output power, consisting of the product Vrms * Irms = P(VA). The input power is by some percentage higher and the difference is due to losses. It is the difference between the open circuit voltage Vo and the rated voltage Vrms . Ri = (Vo-Vrms) / Irms.
- With the rectifier and cap the load current is not sinusoidal. Only when the momentary voltage is higher than the remaining voltage on C plus the 2 diodes Vf, the current starts charging the cap. it reaches the max. before the voltage peak and goes symetrically down to zero. The max. current is a multiple of the average and the rms value is much higher as with a sine. Both are mainly dependent on the ratio of the interiour resistance to the load resistance, not much on the capacitor size. A higher Ri reduces the peak current a lot, since Ipeak is proportional to 1/sqrt(Rload*Ri)
- The required size of the transformer is mainly dependent on the Irms of the current thru its windings, which dissipates in the Ri and heats up the transformer. So a transformer with a very low Ri has not much advantage, since the Ipeak and Irms go up accordingly. The losses in the diode bridge raise too and can be estimated in the datasheet where there are curves for Ploss (Ipeak/Iav). Since this is for a single diode you have to double the ratio and multiply with 4. The rectifier losses plus the losses in the cap Irms^2*ESR plus the output power have to be lower than the transformer rating.
- The ripple voltage is not so much lower with a low Ri, mainly dependent on the cap size.
to be continued