Clamp check

When a flyback’s switch turns off, the magnetizing current passes to the secondary. The current in the leakage inductance cannot: it drives the drain above the input voltage plus the reflected output voltage until a clamp across the primary conducts [Kollman 2024]. The clamp check takes the worst-case operating point and the clamp’s data, all in SI units. It shows:

• the reflected voltage, the leakage energy and the time the leakage current takes to fall to zero;
• the clamp voltage at the actual peak current, and the switch voltage against its rating;
• the clamp dissipation;
• with a TVS, the output voltage the flyback runs up to without a load.

Each result names its equation. The chart shows how the dissipation and the switch voltage trade against the clamp voltage.

What it computes

Enter the largest input voltage and the largest peak primary current at turn-off. If they occur at different operating points, checking them together is conservative. The turns ratio is 1:$n$, where $n$ is the secondary turns divided by the primary turns, and $L_\mathrm{lk}$ is the leakage inductance referred to the primary.

Reflected voltage

While the output diode conducts, the primary winding carries the output voltage plus the diode drop, divided by $n$ [Erickson & Maksimović 2020, Ch. 6]:

$$V_\mathrm{OR} = \frac{V + V_D}{n}$$

$V_\mathrm{OR}$

output voltage reflected to the primary

$V$

output voltage

$V_D$

output-diode forward drop

$n$

turns ratio, secondary/primary

Flyback: output voltage reflected to the primary✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• ideal (lossless) switches and passive components
• output diode modelled as a constant forward drop V_D

Notes

the primary winding voltage while the output diode conducts; the switch then blocks $V_g$ plus this voltage

Source

Erickson & Maksimović 2020, Ch. 6 (Converter Circuits: the flyback's transformer model); derived here

Derivation

step-by-step sympy derivation

Equation id

flyback.V_OR · equations.yaml entry

A clamp must hold a higher voltage than this. Otherwise it conducts whenever the output diode does, and takes the energy meant for the output.

Leakage energy

At turn-off the leakage inductance carries the peak primary current (leakage energy). The clamp takes that energy in every period (leakage power).

Clamp voltage

A TVS starts to conduct at its breakdown voltage $V_\mathrm{BR}$, and its voltage rises with its current. Its datasheet gives the clamping voltage $V_\mathrm{CL}$ at a peak pulse current $I_\mathrm{PP}$. A dynamic resistance in series with the breakdown voltage models the rise between them [STMicroelectronics 2022]:

$$R_D = \frac{V_\mathrm{CL} - V_\mathrm{BR}}{I_\mathrm{PP}}$$

$R_D$

TVS dynamic resistance

$I_\mathrm{PP}$

TVS peak pulse current

$V_\mathrm{BR}$

TVS breakdown voltage

$V_\mathrm{CL}$

TVS clamping voltage at I_PP

Dynamic resistance of a TVS from its datasheet✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• TVS modelled by its breakdown voltage and a constant dynamic resistance

Notes

the resistance that, in series with the breakdown voltage, gives the datasheet's clamping voltage at its peak pulse current; it applies to the datasheet's pulse, since the dynamic resistance depends on the pulse duration

Source

STMicroelectronics 2022, clamping voltage = breakdown voltage + dynamic resistance x peak current, solved for the dynamic resistance at the datasheet's clamping voltage and peak pulse current

Equation id

tvs.R_D · equations.yaml entry

The tool takes the TVS’s voltage at the peak primary current as the highest clamp voltage:

$$V_\mathrm{clamp} = V_\mathrm{BR} + R_D I_\mathrm{pk}$$

$V_\mathrm{clamp}$

clamp voltage

$I_\mathrm{pk}$

peak current

$R_D$

TVS dynamic resistance

$V_\mathrm{BR}$

TVS breakdown voltage

Clamping voltage of a TVS at a given current✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• TVS modelled by its breakdown voltage and a constant dynamic resistance

Notes

a TVS clamps above its breakdown voltage: judge it by its clamping voltage at the actual peak current

Source

STMicroelectronics 2022, clamping voltage from the breakdown voltage and the dynamic resistance

Equation id

tvs.V_clamp · equations.yaml entry

While the leakage current falls to zero, the TVS voltage falls back towards $V_\mathrm{BR}$, so the tool shows the clamp voltage as a range.

An RCD clamp is a diode into a capacitor with a resistor across it. The capacitor holds the clamp voltage, and the resistor dissipates the clamp’s power. The clamp voltage settles where the two balance (the root above $V_\mathrm{OR}$; see the derivation linked under the equation):

$$V_\mathrm{clamp} = \frac{V_\mathrm{OR} + \sqrt{V_\mathrm{OR}^{2} + 2 R_\mathrm{clamp} L_\mathrm{lk} I_\mathrm{pk}^{2} f_s}}{2}$$

$V_\mathrm{clamp}$

clamp voltage

$I_\mathrm{pk}$

peak current

$L_\mathrm{lk}$

leakage inductance (primary)

$R_\mathrm{clamp}$

clamp resistor

$V_\mathrm{OR}$

output voltage reflected to the primary

$f_s$

switching frequency

Clamp voltage of an RCD clamp set by its resistor✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• the clamp holds a constant voltage while it conducts, so the leakage current falls linearly to zero
• RCD clamp: the capacitor's voltage ripple is small and its resistor dissipates the clamp energy
• periodic steady state

Notes

the resistor dissipates the clamp power, $V_\mathrm{clamp}^2/R_\mathrm{clamp}$; a larger resistor gives a higher clamp voltage and a lower dissipation

Source

Kollman 2024, RCD clamp: the trade-off between the clamp voltage and its losses; derived here

Derivation

step-by-step sympy derivation

Equation id

clamp.rcd.V · equations.yaml entry

By this equation a larger resistor gives a higher clamp voltage, and by the dissipation equation below a lower dissipation: the clamp trades its voltage against its losses [Kollman 2024].

Switch voltage

While the clamp conducts, the switch blocks the input voltage plus the clamp voltage:

$$V_\mathrm{DS} = V_g + V_\mathrm{clamp}$$

$V_\mathrm{DS}$

switch voltage while off

$V_\mathrm{clamp}$

clamp voltage

$V_g$

input voltage

Flyback: switch voltage while the primary clamp conducts✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• ideal (lossless) switches and passive components

Notes

after turn-off the leakage inductance drives the drain above $V_g + V_\mathrm{OR}$ until the clamp conducts and holds it at $V_g + V_\mathrm{clamp}$

Source

Kollman 2024, the leakage inductance drives the drain above the reflected output voltage; the clamp limits it

Derivation

step-by-step sympy derivation

Equation id

clamp.Vds · equations.yaml entry

The margin is the switch’s rating minus this voltage, at the highest clamp voltage.

Reset time and clamp dissipation

While the clamp conducts, the magnetizing inductance holds the primary at $V_\mathrm{OR}$. The leakage inductance therefore sees $V_\mathrm{clamp} - V_\mathrm{OR}$, and its current falls linearly to zero [Erickson & Maksimović 2020, Ch. 2]:

$$t_r = \frac{L_\mathrm{lk} I_\mathrm{pk}}{V_\mathrm{clamp} - V_\mathrm{OR}}$$

$t_r$

time for the leakage current to reach zero

$I_\mathrm{pk}$

peak current

$L_\mathrm{lk}$

leakage inductance (primary)

$V_\mathrm{OR}$

output voltage reflected to the primary

$V_\mathrm{clamp}$

clamp voltage

Time the leakage current takes to fall to zero in a flyback's primary clamp✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• the clamp holds a constant voltage while it conducts, so the leakage current falls linearly to zero

Notes

the leakage inductance sees $V_\mathrm{clamp} - V_\mathrm{OR}$ while the clamp conducts; the closer the clamp voltage is to $V_\mathrm{OR}$, the longer the reset, which must fit in the off-time

Source

Erickson & Maksimović 2020, Ch. 2 (Principles of Steady-State Converter Analysis); derived here

Derivation

step-by-step sympy derivation

Equation id

clamp.t_reset · equations.yaml entry

The reset must fit in the off-time. The closer the clamp voltage is to $V_\mathrm{OR}$, the longer it takes; the tool warns when it would take a whole switching period or more. The clamp would then conduct energy meant for the output, and the equations here no longer hold.

Meanwhile the reflected voltage keeps driving current into the clamp. So the clamp absorbs more than the leakage energy, and the closer the clamp voltage is to $V_\mathrm{OR}$, the more it absorbs [Kollman 2024]:

$$P_\mathrm{clamp} = \frac{L_\mathrm{lk} I_\mathrm{pk}^{2} f_s V_\mathrm{clamp}}{2 \left(V_\mathrm{clamp} - V_\mathrm{OR}\right)}$$

$P_\mathrm{clamp}$

clamp loss

$I_\mathrm{pk}$

peak current

$L_\mathrm{lk}$

leakage inductance (primary)

$V_\mathrm{OR}$

output voltage reflected to the primary

$V_\mathrm{clamp}$

clamp voltage

$f_s$

switching frequency

Power dissipated in a flyback's primary clamp✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• the clamp holds a constant voltage while it conducts, so the leakage current falls linearly to zero
• periodic steady state

Notes

the leakage energy per cycle times $V_\mathrm{clamp}/(V_\mathrm{clamp} - V_\mathrm{OR})$: while the leakage current falls, the reflected voltage keeps driving energy into the clamp, the more so the closer the clamp voltage is to $V_\mathrm{OR}$

Source

Kollman 2024, clamp dissipation against the ratio of the clamp voltage to the reflected voltage; derived here

Derivation

step-by-step sympy derivation

Equation id

clamp.P · equations.yaml entry

For a TVS, this equation at the highest clamp voltage is a lower bound, and at $V_\mathrm{BR}$ an upper bound. The table shows both, and their ratio to the leakage power.

Output voltage without a load

With the load removed and no regulation, the output keeps charging. It stops only when its reflected voltage reaches the clamp voltage, and from then on the clamp takes all of the input power. A TVS holds its voltage while it does, so the output stops at:

$$V_\mathrm{o,max} = n V_\mathrm{clamp} - V_D$$

$V_\mathrm{o,max}$

output ceiling set by the clamp

$V_D$

output-diode forward drop

$V_\mathrm{clamp}$

clamp voltage

$n$

turns ratio, secondary/primary

Flyback: open-load output voltage limited by the primary clamp✓ verified by 7 tests

Assumptions, convention and source

Assumptions

• ideal (lossless) switches and passive components
• output diode modelled as a constant forward drop V_D
• fixed duty ratio, no regulation, load removed
• a clamp whose voltage does not depend on the power it absorbs (a TVS or Zener clamp)

Notes

without a load and without regulation the output charges until its reflected voltage reaches the clamp voltage; from then on the clamp takes all of the input power, which it must be rated for. An RCD clamp's voltage rises with that power (the resistor must dissipate it), so this bound does not hold for it

Source

Erickson & Maksimović 2020, Ch. 6 (Converter Circuits: the flyback's transformer model); derived here

Derivation

step-by-step sympy derivation

Equation id

flyback.V_ceiling · equations.yaml entry

The output capacitor and everything on the output must withstand this voltage, and the TVS must be rated for the whole input power, far more than its dissipation in normal operation.

An RCD clamp gives no such bound. Its voltage is where its resistor dissipates the clamp’s power (the power balance behind the RCD equation above), so at no load, with the whole input power to dissipate, the clamp voltage rises far above its loaded value. The output and the switch voltage rise with it. The tool shows no ceiling for an RCD clamp and warns instead: limit the input power at no load, or add a fixed-voltage clamp.

Choosing the clamp voltage

The chart plots the dissipation (logarithmic axis) and the switch voltage against the clamp voltage, from just above $V_\mathrm{OR}$. A low clamp voltage costs dissipation, and a high one costs switch voltage. The switch’s rating sets the highest clamp voltage the switch allows. The shaded band is the range of the clamp entered.

Worked examples

A TVS clamp:

Example flyback with a TVS primary clamp

Symbol	Value	How it is obtained
$V_g$ input voltage	72 V	given
$V$ output voltage	12 V	given
$V_D$ output-diode forward drop	500 mV	given
$n$ turns ratio, secondary/primary	0.25	given
$L_\mathrm{lk}$ leakage inductance (primary)	2 µH	given
$I_\mathrm{pk}$ peak current	2 A	given
$f_s$ switching frequency	100 kHz	given
$V_\mathrm{DS,rated}$ switch voltage rating	150 V	given
$V_\mathrm{BR}$ TVS breakdown voltage	60 V	given
$V_\mathrm{CL}$ TVS clamping voltage at IPP	90 V	given
$I_\mathrm{PP}$ TVS peak pulse current	10 A	given
$V_\mathrm{OR}$ output voltage reflected to the primary	50 V	Flyback: output voltage reflected to the primary
$E_\mathrm{lk}$ leakage energy per turn-off	4 µJ	Energy stored in the leakage inductance at turn-off
$P_\mathrm{lk}$ leakage energy times fs	400 mW	Leakage-energy power
$R_D$ TVS dynamic resistance	3 Ω	Dynamic resistance of a TVS from its datasheet
$t_r$ time for the leakage current to reach zero	400 ns	Time the leakage current takes to fall to zero in a flyback's primary clamp
$V_\mathrm{clamp}$ clamp voltage	66 V	Clamping voltage of a TVS at a given current
$V_\mathrm{DS}$ switch voltage while off	138 V	Flyback: switch voltage while the primary clamp conducts
$P_\mathrm{clamp}$ clamp loss	1.65 W	Power dissipated in a flyback's primary clamp
${P_\mathrm{clamp}}_{\mathrm{max}}$ clamp loss	2.4 W	Power dissipated in a flyback's primary clamp
$V_\mathrm{o,max}$ output ceiling set by the clamp	16 V	Flyback: open-load output voltage limited by the primary clamp

With the peak current of 2 A, the TVS reaches 66 V, well below its datasheet clamping voltage at 10 A. The dissipation lies between 1.65 W and 2.4 W, several times the leakage power of 400 mW.

The same flyback with an RCD clamp:

Example flyback with an RCD primary clamp

Symbol	Value	How it is obtained
$V_g$ input voltage	72 V	given
$V$ output voltage	12 V	given
$V_D$ output-diode forward drop	500 mV	given
$n$ turns ratio, secondary/primary	0.25	given
$L_\mathrm{lk}$ leakage inductance (primary)	2 µH	given
$I_\mathrm{pk}$ peak current	2 A	given
$f_s$ switching frequency	100 kHz	given
$V_\mathrm{DS,rated}$ switch voltage rating	150 V	given
$R_\mathrm{clamp}$ clamp resistor	4.7 kΩ	given
$V_\mathrm{OR}$ output voltage reflected to the primary	50 V	Flyback: output voltage reflected to the primary
$E_\mathrm{lk}$ leakage energy per turn-off	4 µJ	Energy stored in the leakage inductance at turn-off
$P_\mathrm{lk}$ leakage energy times fs	400 mW	Leakage-energy power
$V_\mathrm{clamp}$ clamp voltage	75 V	Clamp voltage of an RCD clamp set by its resistor
$t_r$ time for the leakage current to reach zero	160 ns	Time the leakage current takes to fall to zero in a flyback's primary clamp
$V_\mathrm{DS}$ switch voltage while off	147 V	Flyback: switch voltage while the primary clamp conducts
$P_\mathrm{clamp}$ clamp loss	1.2 W	Power dissipated in a flyback's primary clamp

This resistor sets a clamp voltage whose switch voltage, 147 V, leaves little margin to the 150 V rating. A smaller resistor lowers the clamp voltage, but the dissipation rises. Without a load, this clamp would not hold the switch within its rating at all (see above).

Limitations

• Ideal clamp. The clamp holds a constant voltage while it conducts (for the RCD clamp, the capacitor’s ripple is small), and the leakage current falls linearly. The off-time is not an input: the tool compares the reset time with the whole switching period only. The clamp diode’s reverse recovery, and the ringing of the leakage inductance with the switch-node capacitance after the clamp stops, are not modelled.
• TVS data. The dynamic resistance derived from the datasheet’s $V_\mathrm{CL}$ at $I_\mathrm{PP}$ applies to that datasheet pulse: it depends on the pulse duration, and the breakdown voltage rises with temperature [STMicroelectronics 2022]. Take the datasheet values over the design’s conditions, not the typical ones, and check the TVS’s power rating for its repetitive dissipation.
• Dissipative clamps only. The energy is turned into heat. Clamps that return it to the input or the output are not covered.

Screenshot

The clamp check with the TVS example.