Analysis, Debugging, and Solutions for Radiated Emissions Caused by Diode Reverse Recovery Current

When a diode is forward-biased, electrons are stored in the P‑region and holes are stored in the N‑region; this phenomenon is known as the charge‑storage effect. Under an applied reverse voltage, electrons and holes drift in opposite directions, generating a reverse‑bias drift current, while simultaneously recombining with the majority carriers. Once the concentrations of electrons and holes have decreased significantly, the reverse‑recovery process is complete, and the diode enters the cutoff state.

Analysis of the Causes of Diode Radiation Emission Issues

1.1, Diode Reverse recovery

When a diode is forward-biased, Electronic Electrons are stored in the P‑region, while holes are stored in the N‑region; this phenomenon is known as the charge‑storage effect. When a reverse bias is applied, electrons and holes drift in opposite directions, resulting in a reverse drift current. Electric current At the same time, it recombines with most of the other majority carriers; once the electron and hole concentrations have decreased significantly, the reverse recovery process is complete, and the diode enters the cutoff state.

The amount of charge stored determines the reverse recovery time. The larger the forward current, the greater the charge‑storage density, and the higher the reverse recovery current. Diodes are widely used as freewheeling elements in buck and boost converters, and as rectifying elements in… Switching power supply In the middle.

When operating in discontinuous mode as a freewheeling component, reverse recovery current occurs, and the voltage spikes generated during reverse recovery can impose stress on the diode. In discontinuous-mode operation, the diode’s parasitic… Capacitor Parasitic oscillations caused by parasitic inductance in the loop are also one of the major contributors to radiated emissions.

 

1.2 Analysis of the Causes of Parasitic Oscillations in the Freewheeling Diode of a Buck Converter

When the MOSFET turns on, the parasitic capacitance CB3 of the freewheeling diode is charged, and the parasitic inductances LB3 and LB5 store energy. When the voltage at the SW node equals the input voltage, the energy stored in LB3 and LB5 resonates with CB3 in an LC series circuit, generating overshoot and ringing interference.

Schematic diagram of the freewheeling diode charging current during MOSFET turn-on.

Overshoot and ringing waveform caused by the parasitic capacitance of the freewheeling diode

 

1.3 Analysis of the Causes of Parasitic Oscillations in the Boost Converter’s Boost Diode

When the MOSFET turns on, the parasitic capacitance CB2 of the boost diode is charged, and the parasitic inductances LB1, LB2, LB3, and LB4 store energy. When the voltage at the SW node reaches the input voltage, the energy stored in LB2, LB3, and LB4 resonates in series with CB2, giving rise to overshoot and ringing interference.

Schematic diagram of the charging current through the boost diode when the MOS switch is turned on.

Overshoot ringing waveform caused by the parasitic capacitance of the boost diode

 

1.4、Secondary Side of a Flyback Switching Power Supply Rectifier diode Analysis of the Causes of Parasitic Oscillation

When a rectifier diode turns on or off, it exhibits a very broad spectral content, and the switching frequency and its harmonics themselves constitute significant sources of electromagnetic interference. During the primary-side flyback MOSFET turn‑on and the secondary-side rectifier diode turn‑off, the auxiliary-side magnetizing inductor is clamped, causing the auxiliary-side leakage inductance LES and the diode’s stray capacitance CJ to oscillate. The oscillation frequency is given by:

When the flyback MOSFET turns off, the secondary-side diode switches from conduction to cutoff, and the primary-side magnetizing inductor is discharged. The parasitic capacitances of the CDS and the primary inductor are in parallel, and the resulting oscillatory noise—generated by this parallel combination together with the primary inductor LP (the sum of the magnetizing inductance and leakage inductance)—is coupled through the transformer. Coupling To the secondary side, a common-mode current loop is formed.

 

1.5 Analysis of the Causes of Parasitic Oscillations in the Primary-Side RCD Snubber Circuit of a Flyback Converter

The voltage spikes caused by transformer leakage inductance are directly related to the magnitude of that leakage inductance. The amplitude of these spikes determines the peak charging current through the RCD snubber capacitor; if the current spikes generated during capacitor charging are not properly suppressed, they can lead to severe electromagnetic radiation issues.

To limit the current spikes in the RCD snubber capacitor, a series resistor is added to the RCD snubber circuit. Resistance , which can slow down the capacitor’s charging rate and reduce current spikes, is one of the practical and reliable measures for improving its EMI performance.

In RCD snubber circuits, the diodes operate in switching mode, and their reverse recovery time typically has a significant impact on EMI performance. From the perspective of reverse recovery alone, the longer the reverse recovery time, the smaller the reverse recovery current, and the better the EMI performance; conversely, shorter reverse recovery times lead to poorer EMI performance.

The reverse recovery time of a diode is determined by its parasitic capacitance, which in turn is typically governed by the diode’s package and manufacturing process. For diodes of the same specification from the same manufacturer, fast‑recovery diodes generally have lower parasitic capacitance, while slow‑recovery diodes exhibit higher parasitic capacitance; thus, parasitic capacitance essentially reflects the reverse recovery time. By connecting a capacitor in parallel across the RCD snubber diode, it is possible to mitigate radiation issues caused by the diode’s reverse recovery.

 

Analysis, Debugging, and Solutions for Radiated Emissions Caused by Diode Reverse Recovery Current

When a diode is used as a freewheeling or rectifying element, the magnitude of its reverse recovery current is influenced by the diode’s parasitic capacitance. Once the device type is selected, the parasitic capacitance is essentially fixed; to alter it, an RC snubber circuit must be added across the diode to modify its parasitic capacitance.

The magnitude of the diode’s reverse recovery current is influenced not only by the device selection but also critically by the circuit’s operating mode. When the diode operates in continuous conduction mode or critical conduction mode, the impact of the reverse recovery current can generally be neglected, provided that… Electrical On the basis of performance, diodes operate best in continuous mode.

 

2.1 Diode Device Selection

Select the appropriate diode based on the specific application requirements. Parameter This is critical for the reliable design of electrical performance: the higher the diode’s current‑carrying capacity, the greater its parasitic capacitance may be, which in turn can lead to longer reverse‑recovery times and, when operating in discontinuous mode, lower frequencies of parasitic oscillations.

Although paralleling diodes can increase the current‑carrying capacity, it also increases the parasitic capacitance. Moreover, due to variations in manufacturing processes, individual diodes may exhibit differing electrical parameters, which can lead to uneven current sharing and, over prolonged operation, eventual device failure.

When selecting diodes, in addition to considering the impact of reverse recovery time, attention should also be paid to package considerations. On the basis of meeting performance requirements, surface-mount diodes should be preferred; through-hole diodes can generate significant electromagnetic radiation, which may easily interfere with nearby… Signal Induced noise voltage is generated in the loop.

 

2.2、Adding an RC snubber circuit to the diode to mitigate reverse recovery current issues

Buck freewheeling diode with parallel RC snubber circuit

Boost step-up diode with a parallel RC snubber circuit

Add an RC circuit to the secondary-side rectifier diode of the switch.

RCD snubber diode in parallel with an RC snubber circuit

Once the diode model has been selected, a parallel RC snubber circuit can be added across the diode to mitigate voltage spikes caused by reverse recovery. Many engineers prefer to connect a capacitor directly in parallel with the diode; however, using an RC network is advantageous because capacitors generate current spikes during charging and discharging, and the series resistor helps suppress these transient currents. Furthermore, when a capacitor and parasitic inductance form a parasitic oscillation, the resistor can dampen the resulting LC oscillations.

 

Parasitic Oscillation Caused by Diode Parasitic Capacitance

3.1 Parasitic Oscillations Caused by the Parasitic Capacitance of the Diode in the RCD Snubber Circuit

Use of RCD snubber circuit Schottky Diode waveform

Waveform description: The blue trace shows the drain‑to‑source voltage waveform of the primary‑side MOSFET, the purple trace shows the cathode voltage waveform of the RCD snubber diode, and the green trace shows the current waveform of the primary‑side MOSFET. As observed in the measured waveforms, both the overshoot and ringing on the MOSFET drain are quite pronounced, whereas the overshoot on the RCD snubber diode’s cathode is relatively small.

Waveform when using a slow‑recovery diode in an RCD snubber circuit

Waveform description: The blue trace shows the drain‑side voltage waveform of the primary‑side MOSFET, the purple trace shows the cathode voltage waveform of the RCD snubber diode, and the green trace shows the current waveform of the primary‑side MOSFET. As observed in the measured waveforms, the overshoot and ringing on the MOSFET drain voltage are significantly reduced, the overshoot on the RCD snubber diode’s cathode voltage is also lowered, and the current spikes in the MOSFET are diminished.

Waveform when a 47 pF snubber capacitor is connected in parallel with the diode in an RCD snubber circuit.

Waveform description: By connecting a 47 pF capacitor in parallel across the RCD snubber diode, the overshoot amplitude of the primary-side MOSFET is reduced only slightly, while the cathode voltage overshoot of the RCD snubber diode is significantly attenuated; the current waveform of the primary-side MOSFET remains essentially unchanged.

Waveform when a 30-ohm series resistor is added to the RCD snubber circuit.

By adjusting only the series resistor parameter, the slope of the voltage oscillation waveform at the drain of the primary-side MOSFET can be modified, and the overshoot amplitude is correspondingly reduced. Meanwhile, the cathode‑side voltage overshoot of the RCD snubber diode exhibits only a minor change, whereas its overshoot slope varies more noticeably; in contrast, the current waveform of the primary-side MOSFET shows little variation.

 

3.2 Parasitic Oscillations Caused by the Parasitic Capacitance of the Freewheeling Diode

Radiation Emission Test Data

During the radiated emissions test of a certain product, the emission at the 195 MHz frequency exceeded the standard limit. The excessive emissions exhibited an envelope‑shaped pattern, and broadband interference was conclusively attributed to the switching power supply circuit.

Oscillation waveform at the boost diode

Use a spectrum analyzer to pinpoint the source of noise interference originating from the boost circuit, using… Oscilloscope Measurement of the waveform at the diode’s dynamic node reveals significant oscillations, with an oscillation frequency that closely matches the frequency at which radiated emissions exceed the limits. This strongly indicates that the noise originates from this circuit section.

Analysis of the root cause: At the instant the MOSFET turns on, due to the reverse recovery current, the diode current becomes negative and returns to zero after a certain delay. Subsequently, the diode junction capacitance, together with the MOSFET and… PCB The parasitic inductance of the wiring loop gives rise to an LC oscillation. By measuring the anode voltage and current of the diode, one can observe a distinct oscillatory waveform, as shown in the figure above.

Improving parasitic oscillations can be addressed by optimizing the parasitic capacitance and inductance of the diode. Adding a parallel diode across the diode or incorporating an RC snubber circuit at its terminals can affect temperature rise. Reducing the loop’s parasitic capacitance through an RC network significantly mitigates radiated emissions, while inductance has no impact on thermal performance. Shortening the loop trace length is an effective strategy for minimizing parasitic inductance.

Boost freewheeling diode with added RC snubber circuit

Radiated Emission Test Data After Adding an RC Snubber to the Freewheeling Diode

Parasitic inductance typically arises from PCB trace routing. To reduce parasitic inductance, one can shorten the PCB traces, increase their width to decrease the loop area, and employ high‑frequency bypass capacitors to shunt the current loops, thereby minimizing the inductance associated with the wiring.

Methods for Shortening PCB Routing

Radiated Emission Test Data After Adding a High-Frequency Bypass Capacitor to the Freewheeling Diode Output

Schematic diagram showing the current loop of the boost freewheeling diode and the placement of the high-frequency bypass capacitor.

With the parasitic inductance of the PCB traces and the parasitic parameters of the components already determined, in addition to mitigating parasitic oscillations by adding high‑frequency bypass capacitors, one can also suppress such oscillations by introducing damping into the loop. However, since this is a power‑supply loop, simply increasing resistance is not feasible. Instead, inserting a ferrite bead into the loop—leveraging its high impedance at high frequencies—can effectively dampen parasitic oscillations and achieve the desired result.

Because magnetic beads exhibit inductive characteristics, and inductors cannot abruptly change the current across their terminals, they generate reverse voltage spikes. These voltage spikes must remain within the diode’s rated voltage stress range; otherwise, the diode may experience reverse breakdown due to excessive voltage, leading to… Circuit design It poses a significant potential hazard. Therefore, the selection of magnetic beads must meet EMC On the basis of performance, the smaller the sensitivity, the better.

Add a series ferrite bead before the boost freewheeling diode.

Radiated emission test data after adding a series ferrite bead upstream of the boost freewheeling diode.

Waveform with a series ferrite bead added before the boost freewheeling diode.

Problem Solution:

A parallel RC snubber circuit is connected across the diode; its specific parameters are determined based on actual debugging results. A side effect is that it can affect the diode’s temperature rise.

Reduce parasitic inductance in the loop, i.e., shorten the output lead. High-frequency capacitor The wiring length between the integrated MOS transistor and the reference ground has no adverse effects.

Adding a ferrite bead to the diode’s reverse-recovery loop suppresses parasitic oscillations; however, this also increases the diode’s electrical stress, posing a risk of diode damage.