Causes of high frequency noise in boost DC-DC converter
Time:2024-03-25
Views:215
The key points of this paper
In step-up DC-DC converter, high-speed pulse-like current will flow into the output, which will cause ringing and produce high-frequency noise.
The energy source of ringing when the low-side switch is turned off comes from the conduction delay of the high-side switch and the inductance of the output loop, which makes the voltage of the switch node rise to the potential difference higher than the output voltage.
The ringing when the low-side switch is turned off is caused by the LC resonance caused by the COSS of the low-side switch and the inductance component of the output loop.
The energy source of ringing when the low-side switch is turned on comes from the reverse recovery current of the high-side switch and the charging current of the high-side switch capacitor.
The ringing of the low-side switch is caused by the LC resonance caused by the capacitive component of the high-side switch and the inductive component of the output loop.
This paper introduces the first topic, "Causes of High Frequency Noise in Boost DC-DC Converter".
The causes of high frequency noise much higher than the switching frequency in the output of the step-up power supply will be explained from several angles: the operation of the step-up DC-DC converter, the inductance component in the output capacitor and wiring, the output capacity and ringing of the low-side switch, and the operation when the low-side switch is turned on.
Operation of step-up DC-DC converter
In boost DC-DC converter, when the low-side switch is turned on, the inductance current will increase and energy will be accumulated; When the low-side switch is turned off, the accumulated energy will be released, which will cause the inductor to generate a high voltage caused by back electromotive force, and finally form an output voltage higher than the input voltage. During the operation of the switch, the high-speed change of voltage and current will produce high-frequency vibration, which will become high-frequency noise and be propagated and radiated through the output line, thus causing failure.
When the voltage VSW of the switching node becomes higher than the output voltage, the high-side switching rectifier diode is forward biased. Even if the forward bias voltage is higher than VF of the rectifier diode, the current will not start to flow because of the conduction delay of the rectifier diode, and the drain voltage will rise to a voltage higher than "output voltage +VF". After several ns of conduction delay, the rectifier diode turns on, and the inductor current begins to charge the output capacitor.
Inductive components in output capacitors and wiring
Due to the conduction delay of the rectifier diode, VSW, which has risen to a potential higher than the output voltage, is applied to the output capacitor, and the output capacitor starts to be charged quickly because of the large potential difference. Until then, the output capacitor has been supplying current to the load through discharge, but from now on it will be charged through inductor current. From discharging to charging, the current flowing through the capacitor changes to the opposite direction at nanosecond speed.
The inductance component ESL of the capacitor ranges from several nH to several tens of NH, but according to the formula ΔV =ΔI× L/ΔT composed of the change value ΔI (in a unit) and the change speed ΔT (in ns unit) of the charging and discharging current, back electromotive force Δ V will occur within Δ T, and high voltage will also be generated in the output capacitor. Therefore, the output capacitor will produce a spike-like high voltage immediately after the rectifier diode is turned on.
Output capacity and ringing of low-side switch
COSS is charged to the point where a spike-like high voltage will also be generated in the output capacitor, and then the energy stored in COSS is released and flows into the output capacitor, and is stored in the inductive component ESL in the form of magnetic energy.
When the voltage of COSS drops to the potential of VOUT through discharge, the potential difference disappears, and the magnetic energy in ESL stops increasing. Using the magnetic energy previously accumulated by ESL, it enters a constant current state and continues to draw charges from COSS, and the voltage of COSS drops below VOUT.
With the decrease of COSS voltage, the magnetic energy of ESL decreases, and when the magnetic energy reaches 0, COSS stops discharging. At this time, the potential of COSS is lower than VOUT, and the current flows reversely from the output capacitor to the COSS of the low-side switch, charging COSS, and at the same time, the magnetic energy in the opposite direction is accumulated in ESL.
After that, the energy moves repeatedly between the COSS and the output capacitor ESL of the low-side switch, and an oscillation state is generated due to the reciprocation of voltage and current. Strictly speaking, the inductance component is the LC resonance caused by the sum L of the ESL and the inductance component of the loop flowing through the charging and discharging current and the COSS of the low-side switch, and the resonance frequency FZ is FZ=1/2π√(L×COSS).
The inductance component totals several nH, and the capacity of COSS is tens to hundreds of pF, so the resonance frequency is usually in the range of tens to hundreds of MHz. Ringing at the switch node and output capacitor will become high-frequency noise, which will spread through the power output line and cause faults such as misoperation of load circuit.
Operation of low-side switch when it is turned on
Just before the low-side switch is turned on, the voltage of "output voltage+VF" will be applied to the drain of the low-side switch, and the inductor current will flow in the rectifier diode (i.e. the high-side switch). From here on, when the low-side switch transitions to the conduction state, the inductor current flowing to the output will flow from the high-side switch to the low-side switch.
The voltage of VSW begins to drop to GND potential, and the rectifier diode (high-side switch) becomes reverse biased. Because the current flowing through the diode before this makes free electrons and holes exist at the junction of the diode, even if the diode is in the reverse bias state, it will not immediately turn off, and a reverse current will flow in a short time. This current is called "reverse recovery current", which makes the output loop flow with a reverse current whose current changes at a high speed.
When the reverse recovery current ends, the diode turns off due to the reverse bias state, and passes through the depletion layer of the PN junction to become a capacitor with small capacitance. When the high-side switch adopts synchronous rectification mode of FET, FET has parasitic diode, so there is a capacitor COSS formed by "depletion layer+physical structure between source and drain+gate capacitance". The charging current will flow to these capacitors with the reverse recovery current. In the LC resonant circuit composed of the capacitance of the high-side switch and the inductance component of the output loop, high-frequency noise with reverse current as the energy source will be generated.
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