Flyback Regulator with LT1070


The flyback converter is based on the buck-boost converter.



Flyback regulators use a transformer to transfer energy from input to output. When S1 is "on", dot ends of all windings are negative with respect to their no-dot ends. During this time, energy builds up in the core due to increasing current in the primary winding. At this time, the polarity of the output winding is such that D1 is reverse biased. When S1 opens, the total stored energy is transferred to the secondary winding and current is delivered to the load. The turns ratio (N) of the transformer can be adjusted for optimum power transfer from input to output. The output load currents are supplied from storage filter capacitor C.

Peak switch current in a flyback regulator is equal to:

Notice that peak switch current can be reduced to a minimum by using a very small value for N. This has two negative consequences however; the switch voltage and diode current become very large during switch off time. For a given maximum switch voltage, optimum power transfer occurs at VIN = 1/2VMAX.

Enter the given values





Both input ripple current and output ripple current are high in a flyback regulator, but this disadvantage is more than offset in many cases by the ability to achieve current or voltage gain and the inherent isolation afforded by the transformer. Output voltage is given by:

Enter the given values





With any value of N, a duty cycle between 0 and 1 can be found which generates the required output. Flyback regulators can have an output voltage which is higher or lower than the input voltage. A disadvantage of flyback regulators is the high energy which must be stored in the transformer in the form of DC current in the windings. This requires larger cores than would be necessary with pure AC in the windings.

Notice that peak switch current can be reduced to a minimum by using a very small value for N. This has two negative consequences however; the switch voltage and diode current become very large during switch off time. For a given maximum switch voltage, optimum power transfer occurs at VIN = 1/2VMAX.

Enter the given values




Flyback converters are able to regulate an output voltage either higher or lower than the input voltage by shuttling stored energy back and forth between the windings of a transformer. During switch "on" time, all energy is stored in the primary winding according to: E = (IPRI)2(LPRI)/2. When the switch turns off, this energy is transferred to the output winding. The current in the secondary just after switch opening is equal to the reciprocal of turns ratio (1/N) times the current in the primary just prior to switch opening. Output voltage of a flyback converter is not constrained by input voltage as in buck or boost converters.

By varying duty cycle between 0 and 1, output voltage can theoretically be set anywhere from 0 to ∞. Practically, however, output voltage is constrained by switch breakdown voltage and the maximum output voltage is limited to:

VSNUB = snubber voltage (see snubber details in this section)

VM = maximum allowed switch voltage = 60V (LT1070)

Enter the given values




This still allows the LT1070 to regulate output voltages of hundreds or even thousands of volts by using large values of N.
In many applications, N can vary over a wide range without degrading performance. If maximum output power is desired however, N can be optimized:

Enter the given values




A second important transformer parameter which must be determined is primary inductance (LPRI). For maximum output power, LPRI should be high to minimize magnetizing current, but this can lead to unacceptably large core sizes. A reasonable design approach is to reduce the value of LPRI to the point where primary magnetizing current (ΔI) is about 20% of peak switch current. The LT1070 is rated for 5A peak switch current, so for full power applications, ΔI can be set to 1A peak-to-peak. Maximum output current is reduced by one-half of the ratio of ΔI to peak switch current, or ≅10% in this case.

With this design approach, LPRI is found from:

Enter the given values





Values of LPRI higher than this will raise maximum output current only slightly and will require larger core size. Lower primary inductance may be used for lower output currents to reduce core size.

Maximum output current is a function of peak allowed switch current (IP):

IP = maximum LT1070 switch current
E = overall efficiency ~ 75%

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The 75% efficiency number comes from losses in the snubber network (~6%), LT1070 switch (~4%), LT1070 driver (~3%), output diode (≈8%) and transformer (~4%). Although this efficiency is not as impressive as the 85% to 95% obtainable with simple buck or boost designs, it is more than justified in many cases by the ability to use the variable N to generate high output currents or high output voltages and the option to add extra windings for multiple outputs.

Peak primary current is used to determine core sizing for the transformer:

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The core must be able to handle xxxx A peak current in the xxxx uH primary winding without saturating.



Output Divider

R1 and R2 set output voltage:





Frequency Compensation

R3 and C2 provide a pole-zero frequency compensation. For details, see the section on frequency compensation elsewhere in this application note.




Snubber Design

Flyback converters using transformers require a clamp to protect the switch from overvoltage spikes. These spikes are created by leakage inductance in the transformer. Leakage inductance (LL) is modeled as an inductor in series with the primary winding which is not coupled to the secondary as shown in Figure below.

During switch "on" time, a current is established in LL equal to peak primary current (IPRI). When the switch turns off, the energy stored in LL, (E = I2pri * LL/2) will cause the switch voltage to fly up to breakdown if the voltage is not clamped.


If a Zener diode is used for clamping, Zener clamp voltage is selected by assigning a maximum switch voltage and maximum input voltage:



If a Zener diode is used for clamping, Zener clamp voltage is selected by assigning a maximum switch voltage and maximum input voltage:

VZENER = VM – VIN(MAX)


VM = maximum allowed switch voltage



The standard LT1070 maximum switch voltage is 65V, so VM is typically set at 60V to allow a margin of 5V. If we use VIN(MAX) = xxxV entered above, for this circuit:





Peak Zener current is equal to peak primary current (IPRI) and average power dissipation is equal to:



An important part of this equation is the term [VZ – (VOUT + VF)/N] in the denominator. This voltage is defined as snubber voltage (VSNUB) and is the difference between the Zener voltage and the normal flyback voltage of the primary. (See waveforms with Figure 22.) If VSNUB is too low, Zener dissipation rises rapidly. A reasonable minimum for VSNUB is 10V.

Leakage inductance in a transformer can be minimized by bifilar winding or by interleaving the primary and secondary. If this is done correctly, leakage inductance is usually less than 1% of primary inductance. If we wind T1 for LPRI = 230µH, LL should be less than 2.3µH.

Zener dissipation under short-circuit conditions is calculated from the same equation by assuming that VOUT = 0V and IPRI is the current limit value of the LT1070. If we let IPRI = 9A:



An alternative to Zener clamping is an R/C clamp. This is less expensive, but has the disadvantage of a less welldefined clamping level. The RC snubber also dissipates power even with no-load conditions. A value for R4 is found from: (p33)

VR = voltage across snubber resistor.

If we set VR = ??? (same as VZENER) and use full load conditions of IPRI = ???:




Power dissipation in the snubber at full load is equal to: