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1. Flyback Converter Theory
2. Continuous Mode Photography Definition
3. Flyback Converter Waveforms

The flyback converter is an isolated power converter. The two prevailing control schemes are voltage mode control and current mode control (in the majority of cases current mode control needs to be dominant for stability during operation).

The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between the input and any outputs. The flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. When driving for example a plasma lamp or a voltage multiplier the rectifying diode of the boost converter is left out and the device is called a flyback transformer.

Structure and principle[edit]

1. Continuous mode is possible in a flyback converter, but it gets tricky. If you have to ask basics here, don't try to do continuous mode. A trick I often use at this point is to set up a PWM output of a microcontroller to produce the switching pulses.
2. The “continuous conduction mode flyback transformer” presents us with a more difficult design challenge. This stems from the fact that flyback transformers are not really transformers. In fact, in the continuous flyback mode, the transformer is more correctly a choke carrying ac and dc currents.

The schematic of a flyback converter can be seen in Fig. 1. It is equivalent to that of a buck-boost converter,^[1] with the inductor split to form a transformer. Therefore, the operating principle of both converters is very similar:

• When the switch is closed (top of Fig. 2), the primary of the transformer is directly connected to the input voltage source. The primary current and magnetic flux in the transformer increases, storing energy in the transformer. The voltage induced in the secondary winding is negative, so the diode is reverse-biased (i.e., blocked). The output capacitor supplies energy to the output load.
• When the switch is opened (bottom of Fig. 2), the primary current and magnetic flux drops. The secondary voltage is positive, forward-biasing the diode, allowing current to flow from the transformer. The energy from the transformer core recharges the capacitor and supplies the load.

The operation of storing energy in the transformer before transferring to the output of the converter allows the topology to easily generate multiple outputs with little additional circuitry, although the output voltages have to be able to match each other through the turns ratio. Also there is a need for a controlling rail which has to be loaded before load is applied to the uncontrolled rails, this is to allow the PWM to open up and supply enough energy to the transformer.

Operations[edit]

The flyback converter is an isolated power converter. The two prevailing control schemes are voltage mode control and current mode control (in the majority of cases current mode control needs to be dominant for stability during operation). Both require a signal related to the output voltage. There are three common ways to generate this voltage. The first is to use an optocoupler on the secondary circuitry to send a signal to the controller. The second is to wind a separate winding on the coil and rely on the cross regulation of the design. The third consists of sampling the voltage amplitude on the primary side, during the discharge, referenced to the standing primary DC voltage.

The first technique involving an optocoupler has been used to obtain tight voltage and current regulation, whereas the second approach has been developed for cost-sensitive applications where the output does not need to be as tightly controlled, but up to 11 components including the optocoupler could be eliminated from the overall design.^{[citation needed]} Also, in applications where reliability is critical, optocouplers can be detrimental to the MTBF (Mean Time Between Failure) calculations. The third technique, primary-side sensing, can be as accurate as the first and more economical than the second, yet requires a minimum load so that the discharge-event keeps occurring, providing the opportunities to sample the 1:N secondary voltage at the primary winding (during Tdischarge, as per Fig3).

A variation in primary-side sensing technology is where the output voltage and current are regulated by monitoring the waveforms in the auxiliary winding used to power the control IC itself, which have improved the accuracy of both voltage and current regulation. The auxiliary primary winding is used in the same discharge phase as the remaining secondaries, but it builds a rectified voltage referenced commonly with the primary DC, hence considered on the primary side.

Previously, a measurement was taken across the whole of the flyback waveform which led to error, but it was realized that measurements at the so-called knee point (when the secondary current is zero, see Fig. 3) allow for a much more accurate measurement of what is happening on the secondary side. This topology is now replacing ringing choke converters (RCCs) in applications such as mobile phone chargers.

Limitations[edit]

Continuous mode has the following disadvantages, which complicate the control of the converter:

• The voltage feedback loop requires a lower bandwidth due to a right half plane zero in the response of the converter.
• The current feedback loop used in current mode control needs slope compensation in cases where the duty cycle is above 50%.
• The power switches are now turning on with positive current flow - this means that in addition to turn-off speed, the switch turn-on speed is also important for efficiency and reducing waste heat in the switching element.

Discontinuous mode has the following disadvantages, which limit the efficiency of the converter:

• High RMS and peak currents in the design
• High flux excursions in the inductor

Applications[edit]

• Low-power switch-mode power supplies (cell phone charger, standby power supply in PCs)
• Low-cost multiple-output power supplies (e.g., main PC supplies <250 W^{[citation needed]})
• High voltage supply for the CRT in TVs and monitors (the flyback converter is often combined with the horizontal deflection drive)
• High voltage generation (e.g., for xenon flash lamps, lasers, copiers, etc.)
• Isolated gate driver

See also[edit]

• Joule thief - Minimalist switchmode converter example

References[edit]

• Billings, Keith (1999), Switchmode Power Supply Handbook (Second ed.), McGraw-Hill, ISBN0-07-006719-8

1. ^The Flyback Converter - Lecture notes - ECEN4517 - Department of Electrical and Computer Engineering - University of Colorado, Boulder.

I am trying to design an isolated dc dc converter. Now, my input is approx 20V and I need an output of 7V at the final output. I understand that I need a PWM controller that is approx 100kHz or so. Now, how do I control the duty cycle of such an IC ? I need 7V from a 20V supply. So approx its 35% duty cycle. Also, I need a transformer for the isolation. How shall I select such a transformer ? I know this is a huge topic(selecting a transformer), but any pointers will be appreciated. Also, will I need a feedback or such ?

^{simulate this circuit – Schematic created using CircuitLab}The above is a representative schematic of this circuit.Now, regarding the PWM generator, can an IC 555 suffice ? Will it give out sufficient gate drive ? My main concern is regarding the selection of the transformer.

The load is a max of 4Amps.

closed as too broad by Adam Lawrence, PeterJ, Daniel Grillo, nidhin, Brian DrummondFeb 14 '16 at 22:13

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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1 Answer

In the question you say you want 4 A at 7 V (28 W), but in a comment 5 V at 10 A (50 W). That rules out cheap and available POE (power over Ethernet) transformers, which I often use for such things. Flyback transformers at the power levels you want are more scarce.

In any case, here are roughly the steps:

1. Pick a transformer. Start with the power requirement, then something that has roughly the right input to output voltage ratio.
2. Read transformer datasheet carefully.
3. Look at the primary inductance and compute the switch on time to reach saturation with your input voltage.
4. Look at the secondary inductance and compute the initial output current at the start of the output pulse, and how long that pulse will last given your output voltage.
5. From the previous two points, you know how long a complete switching cycle should take, and how much energy will be delivered per cycle. Verify that you can still get the power you want with some margin. If not, go back to step 1.
6. Pick a PWM period and duty cycle that should transfer a bit more power than you really need, but doesn't violate the saturation limits. You can set the on time right at the saturation limit, but it's good to leave a little more off time to make sure the energy in the core is really zero when the next pulse is started. Continuous mode is possible in a flyback converter, but it gets tricky. If you have to ask basics here, don't try to do continuous mode.

A trick I often use at this point is to set up a PWM output of a microcontroller to produce the switching pulses. If left open loop, this would make a little more voltage than you need under full load.

The trick is to use a shutdown input of the PWM module. Many micros have these. On the isolated side, drive a opto-coupler when the voltage hits the regulation threshold. On the primary side, this activates the shutdown input of the PWM. Make sure the PWM recovers when the shutdown signal is no longer asserted. Basically, that kills the oscillations when the voltage gets to where you want it, then resumes them when it drops below the threshold. This all happens in hardware with 0 CPU intervention after initialization.

This method results in a little more ripple than something that carefully and smoothly adjusts the PWM duty cycle in a control loop, but its a lot simpler and very robust.

I'm doing this in one of my current projects. In this case I need isolated 5 V for some communication interfaces. I use a PNP transistor around a 5 V LDO to sense when the LDO input is the B-E drop above its output. 700 mV or so is a nice comfortable headroom for this LDO. The transistor ultimately drives the feedback opto, which then shuts down pumping power into the isolated section. There is about 100 mVpp ripple on this input to the LDO, but the output is quite flat.

Here is a snippet of a schematic that implements what I described above:

Q5 is the switch, which is driven from the PWM output of the micro via a FET gate driver. The basic power supply on the isolated side is the secondaries of TR3, D6, and C23.

IC7 is the LDO that makes the clean and regulated 5 V power. Q6 is connected in such a way that it turns on when the input of the LDO gets to about 700 mV above its output. That in turn turns on opto IC8, which activates the shutdown input to the PWM, which stops dumping power to the isolated section. Eventually the voltage drops to where the LED in the opto is no longer on enough to keep the PWM off, and power is again transferred to the isolated section.

The input of the LDO has about 100 mVpp ripple on it, which is cleans up nicely to make its regulated output of 5 V.

Flyback Converter Theory

Note that you can sometimes use the rougher 5.7 V supply directly. In this case I have a couple of LEDs, which actually take more power than most everything else. I have the LEDs connected to their current comes from the 5.7 V supply, thereby requiring less current out of the LDO. The extra 700 mV gets wasted as heat either way, but this way distributes it instead of concentrating it in the LDO.

Continuous Mode Photography Definition

Flyback Converter Waveforms

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