Updated : 03/26/2020
One can use this Mathcad file to analyze an Active Clamp Flyback (ACF) converter that operates both in active clamp mode and discontinuous conduction mode.
When a Flyback converter is operated in active clamp mode, the magnetizing current is always in continuous conduction mode. Therefore, it is easy to analyze an active clamp Flyback as an ideal classical Flyback with synchronous rectification. When true synchronous rectification is employed in light load conditions, the magnetizing current traverses both the first and the third quadrant similar to an active clamp Flyback.
The Yellow Boxes are Calculated User Recommended values
The green Boxes are user inputs
The White Boxes are Definition or Equation
If the user would like to examine the calculations in more depth each section expands and has reference if they were used.
Data Sheet Paramters
Definition of milli ohm
Forward Diode Voltage drop
Line based OPP levels
Specifications
Enter the Typical output Voltage Desired and the Tolerance
Note: This may result in higher
power than specced on some
graphs.
T
Adjust Dmax slightly to achieve a turn's ratio that is mechanically viable design.
Sweep of the turns ratio
Voltage spikes permitted on the secondary side.
General switch derating used to set limits
Voltage rating of secondary side switch
Primary side switches maximum voltage rating
The stress on the primary and secondary side switches is calculated using the selected turns ratio
Primary side switch voltage stress
Secondary side switch voltage stress
The derated voltage stress from the selected MOSFET rating
Coss value from data sheet graph belwo 10 V the switch will turn on .
From FDMS86255 Datasheet
Secondary side MOSFET Coss
The driver is connected to the switch node and depending on the technology used the capacitance can be significant thus an accounting of it must be made
The transformer capacitance is added to the capacitance total
General equation for CCM Flyback with added duty ratio for losses
Suggested RT targeted frequency
Suggested RT resistor value
RT resistor selected in this section
Since the Clamp capacitor goes through a complete charging an discharging cycle at
Solving for the clamp capacitor
Nominal DC voltage applied to the
The temperature derating
and
solving for RS
Or in the case of Current Sense Transformer:
Place Diode from the dot of the CT to the CS pin
Place resistor across CT directly (10k)
Rburden Calculation follows:
From the above graph is is clear that the lowest required Current sense resistor value occurs at low line
The current sense resistor selected
The resistor selected varies with the input voltage and current since the OPP compensation is not constant
External OPP
For DC to DC
The OPP voltage induced on the CS pin and its corresponding effect on the COMP voltage.
Th
The RS resistor selected in a different section
Internal ATH thresholds
The FB voltage is plotted for the minimum to maximum output voltage.
These resistances will yield the appropriate bin for optimal transition at the selected current cross over.
Number of turns on the winding used to create the additional bias for transitions
The ratio of aux winding voltage to the secondary voltage
RZ1 is chosen such that no external bias currents effect the transition
If no zener is used
With Zener Off
Solving for R1
With Zener on
Solving for RZ1
If the value is negative it is not a good solution and the Zener must be used you can place a
We will set the Zener Voltage and calculate the Zener Voltage
For
Transition prediction
T
Set internally
The DCM RMS current
The ACF RMS current
Resonant Frequency
Peak current of low side switch
High side drive time
The on time is calculated for ACF mode.
The peak current is calculated
The peak current is plotted for 5- 20V
The valley current is plotted for 5- 20V
The RS resistor selected in a different section
The FB voltage plotted
This is the output Current of transition that the converter should be changing from DCM to ACF.
The DTH pin has a precise pull up current of 16 uA
On the primary side of the converter an aux winding can be used to look at the approximate value of the output voltage an example is provided below
This is the number of turns on the secondary side.
Number of turns on the winding being used to create the additional bias for transitions
This is the ratio of aux winding voltage to the secondary voltage
The coefficients provided below are for the Hitachi Magnetic material ML29D 50 kHz to 250 kHz
The coefficients provided below are for the Hitachi Magnetic material ML29D 50 kHz to 250 kHz
Standard core parameters
Bobbin fro RM7
Bobbins outer diameter where the core fits around the outside
Bobbin wind window
Distance between the core and the windings
The switching frequency in ACF mode it is variable but the maximum delta B occurs at the maximum load, minimum input voltage and maximum output voltage
ACF On time
Maximum Current in ACF mode
This is the maximum normal current for the transformer
OCP Level
Current Sense Resistor
(prior input)
Center leg gap needed to achieve desired inductance with number of turns
Maximum output voltage for variable output designs
Voltage where the lower winding will take over regulation
Lowest output voltage
Drop out voltage of the linear regulator used in the design (if a Zener diode and BJT or MOSFET is used this the threshold voltage which will vary with temperature.
The margin on top of VCCoff so that the converter does not shut down due to transients and variations
Diode Voltage rating
Test frequencies for all of the operating range
Admittance of copper
The relative permeability of copper or other conductors used for winding the transformer
Since the AWG selection varies with the frequency it would be more advantageous to vary the wire gauge and see the result of the variation and power losses associated.
Wire diameter calculated from the AWG provided
The calculated area of the wire
This is the number of wires that will fit into the window for each AWG in one layer.
We could allocate all of the window to windings but
This is the number wires that will fit stacked vertically into the space provide.
Total number of wires that can fit into the pre-allocated space of the selected wire AWG.
Total number of windings is divided by the number of required windings giving the number of parallel wire with the same AWG that can be used in the area (result was rounded).
The number of wires that can fit inside one layer of windings is compared against the parallel windings that will fit into the space giving the number of turns per layer.
Total layers used in making all of the turns on the primary side.
The general parallel and stacking of the windings results in this AC resistance if the wires are not litz wire
The DC resistance of the selected AWG and available space filled with parallel wires gives the DC resistance of the
The resistance of the wires both AC and DC if placed in parallel with all of the layers.
Tape thickness
Number of primary to primary or secondary to secondary interfaces
Auxiliary winding reserved for winding
Number of primary to secondary interfaces
This is the turns per layer with the number of parallel wires used.
The DC resistance of the secondary side with parallel wires
The AC and DC resistance of the secondary side
The AC and DC power losses of the Transformer
AUX winding
Maximum RMS AC Line Current:
Power Dissipation for the Fuse
Typical fuse resistance
Maximum fuse dissipation
Maximum fuse dissipation over all line conditions
This is the thermal rise of the fuse
Ripple Current Specified
Identification of the Minimum ACF Frequency
Calculated minimum capacitance for specified ripple current
Selected Capacitance
Selected Dissipation factor
Calculated ESR from Dissipation factor
Selected ESR
Calculated Ripple current from ESR
Power dissipated in the input capacitors
Snubber Calculations
Primary Side Clamp Resistor
Note: Assumed 20x Fsw_max for
spike frequency.
Note: Whenever transitioning from DCM to ACF, we have to clamp that.
The RClamp is standard as 1-10Meg recommended
The body diode helps with power dissipation.
we assuem the worst case ripple comes from the DMC operation
It is clear from the above plot that the impedance
Selected ESR
Selected Output capacitance
Parallel Ceramic
Since the HS MOSFET does not turn on for the first 600 μs and the frequency is at half the ACF switching Frequency then the average frequency for the HS driver is the following.
To determine the capacitance needed the current drawn is compared to the soft start time
If ceramic capacitors are used they must be derated for bias and temperature.
The temperature derating
Electrolytic Thermal Derating
HV Startup
HV Startup
Static loss for the HV pin also used in light load
VCC regulation voltage
Boot quiescent current.
NCP51530 Boot off threshold
The general rule of thumb is to make boot capacitor 10 X the capacitance of the gate capacitance connected to the driver.
The on time ( Toff for the ACF) of the drive is not the full switching period but we approximate it as that for margin and ease of calculation.
The maximum drive voltage of the driver is 20V any voltage over this will result in the destruction of the NCP51530 but the gate charge is generally specified at 10V.
Capacitance calculated to 10 V but the maximum voltage is at 20V thus if Q is linear then the capacitance at 10 V is half of the maximum capacitance of the application.
Total Gate Charge
Drive Voltage
Ripple voltage on the Boot pin
Boot Capacitor Derating
power loss
Conduction losses for the low side switch in ACF mode
Conduction losses for the high side switch in ACF mode
Secondary side conduction losses
Losses for current sense resistor
Transformer losses
Output capacitor losses
Clamp capacitor losses
Comparison of power loss budget to calcualted power losses
[1] T. LaBella, B. York, C. Hutchens, J. Lai, "Dead Time Optimization through Loss Analysis of an Active-Clamp Flyback Converter Utilizing GaN Devices", IEEE ECCE, 2012.