Hameg HM 7042-4 Manual

Contents

Hameg HM 7042-4 Manual Manual pdf 21 page image

21Subject to change without noticeBasics of power suppliesLinear power suppliesLinear regulated power supplies excel by their highly constantoutput voltage, low ripple and fast regulation, even under high lineand load transients. Good power supplies feature a ripple of lessthan 1 mV rms which is mostly neglegible. Further they are freefrom EMI emission in contrast to SMPS.A conventional mains transformer isolates the line from thesecondary which is rectified and supplies an unregulated voltageto a series pass transistor. Capacitors at the input and output ofthe regulator serve as buffers and decrease the ripple. A highprecision reference voltage is fed to one input of an amplifier, thesecond input is connected mostly to a fraction of the outputvoltage, the output of this amplifier controls the series passtransistor. This analog amplifier is generally quite fast and is ableto keep the output voltage within tight limits.Switched-mode power supplies (SMPS)SMPS operate with very much higher efficiencies than linearregulated power supplies. The DC voltage to be converted ischopped at a high frequency rate thus requiring only comparativelytiny and light ferrite chokes or transformers with low losses, also,the switching transistor is switched fully on and off henceswitching losses are low. In principle regulation of the outputvoltage is achieved by changing the duty cycle of the switchdriving waveform.Off-line SMPSThe line voltage is rectified, the buffer capacitor required is offairly small capacitance value because the energy stored isproportional to the voltage squared (E = 1/2 x C x U 2).Secondary SMPSThese still require a 50 or 60 Hz mains transformer, the secondaryoutput voltage is rectified, smoothed and then chopped. Thecapacitance values needed here for filtering the 100 resp. 120 Hzripple are higher due to the lower voltage.All SMPS feature a very much higher efficiency from appr. 70 up toover 95 % compared to any linear supply. They are lighter, smaller.ACvoltageswitchingtransistorrectifier filter outputDCvoltageGNDmainstransformerOPVAcontrolTRD TThe capacitors on the output(s) of a SMPS may be quite small due tothe high frequency, but the choice depends also on other factors likeenergy required for buffering or ac ripple from the load (e.g. motors).In principle the size of the major components decreases withincreasing operating frequency, however, the efficiency dropsapppreciably above appr. 250 kHz as the losses in all componentsrise sharply.Parallel and series operationIt is mandatory that the power supplies used are definitelyspecified for these operating modes. This is the case with allHAMEG supplies. As a rule, the output voltages to be combinedare independent of each other, hence, it is allowed to connect theoutputs of one supply with those of another or more.Series operationIn this mode the output voltages add, the output current is thesame for all supplies. As the sum of all voltages may well surpassthe 42 V limit touching of live parts may be fatal! Only qualified andwell instructed personnel is allowed to operate such installations.Parallel operationBasics of power suppliesTT2D Q2D Q1D IACvoltagemains transformer rectifier actuatoranalog control outputreference voltageREFDCvoltageGNDC1 OPVA C2B1TR1Power supply 1Output 1+ -Power supply 12Output+ -Power supply 21Output+ -U 3U 2U 1U = U + U + Utotal 1 2 3I = I = I = Itotal 1 2 3I2 I3I1+ - + - + -U = U = U = U1 2 3totalI = I + I + Itotal 1 2 3U 3U 2U 1I2 I3I1Power supply 1Output 1Power supply 12OutputPower supply 21OutputACvoltagemainsrectifier rf-transformer rectifier filter outputpotential seperationDCvoltageGNDswitchingtransistorscreeningOPVAOCBcontrol