Method of measuring ripple of switching power supply

1、 Estimate the output ripple in the step-down regulator

        For output ripple calculation under steady-state conditions , Do not ignore the main flow of electricity Rong C Inductance current ripple . chart 3 Display diagram 2 Middle switch position 1 And switch position 2 Corresponding switching node voltage and inductance current waveform . Inductance current waveform iL (t) Including DC current (ILOAD) Current ripple with peak to peak amplitude ∆IL.

        The DC current only flows into the load resistor RLOAD, Because capacitors block all direct current Flow through it and out . Inductance current ripple in capacitance C And load resistance RLOAD Apart from each other . But the capacitance must be big enough , So that it is at the switching frequency The impedance under this condition is obviously lower than that of the load . then , Because most of the inductance current ripple flows through the capacitor C, The capacitor will provide enough switching ripple filtering .

chart 4 The capacitive output ripple from the buck converter is shown . Capacitor output ripple Wave and capacitive current waveform above zero reference level iL (t) The positive part of contains electricity Charge correlation . The waveform of capacitance current is the same as that of inductance current , But no ILOAD composition . The capacitive current waveform is maintained for half the time during switching Hold above zero reference level , This makes the capacitor voltage VC (t) growth , Because the charge is being stored on the capacitor board . The capacitive current waveform is below zero reference level when ,VC (t) It will lower . Therefore, between two zero crossing events of capacitive current Is the switch position 1 And switch position 2 Half of , This will cause the capacitor voltage to fall below The variation between its minimum and maximum values .

 

 

According to the figure 4 Calculate the output ripple size based on the current waveform on the capacitor ：

The amount of charge on a capacitor Q=C*V

The amount of charge on a capacitor Q=I*T

C*V= 1/2 (1/2 * Fsw * ∆I / 2) = ∆I / (8 * Fsw）

∆V = ∆I / (8 * Fsw * C）

among ：

∆v Is the capacitor voltage ripple .

∆I  Is the inductance current ripple .

Fsw Is the switching frequency .

C For capacitance .

Example ：

For the calculation of output ripple here, please refer to my article DCDC Inductance calculation article , There is a detailed introduction .

 

2、 Oscillograph method for measuring ripple

1> The following is the wrong way to measure ripple ：

2、 Still the right way

The probe shall be connected to the grounding ring , Set bandwidth limit for oscilloscope 20M.

 3、 The right way ：

The best way is to use 50 Ω Coaxial cable , The cable is maintained at 50 Ω In the environment , And through the optional 50 Ω The input impedance of oscilloscope is terminated . At the switching regulator output A DC isolating capacitor is placed between the capacitor and the input end of the oscilloscope . Another of the cables The terminal can be directly welded to the output capacitor through a very short flying wire ( See chart 12 and chart 13). This keeps the signal integrity , Especially in the wide bandwidth range When the signal with very low level is measured within the range .

  Reference article ：

ADI Of AN-1144 Apply notes