As a result of their versatility, ease of design, and low value, flyback converters have develop into one of the extensively used topologies in energy electronics. Its construction derives from one of many three primary topologies—particularly, buck-boost topology. Nonetheless, in contrast to buck-boost converters, flyback topologies enable the voltage output to be electrically remoted from the enter energy provide. This function is significant for industrial and shopper functions.
Among the many completely different management strategies used to stabilize energy converters, probably the most extensively used management technique is peak present mode, which constantly senses the first present to offer necessary safety for the ability provide.
Moreover, to acquire a better design efficiency, it’s frequent to manage the converter with the output that has the very best load utilizing a method known as cross-regulation.
This text goals to indicate engineers learn how to accurately design the management loop that stabilizes the flyback converter with a purpose to present optimum performance. This course of contains minimizing the stationary error, growing/reducing the bandwidth as required, and growing the section/achieve margin as a lot as doable.
Closed-loop flyback converter block diagram
Earlier than making the required calculations for the controller to stabilize the height present management mode flyback, it’s necessary to know the parts of your entire closed-loop system: the converter averaged mannequin and the management loop (Determine 1).
Determine 1 Right here is how the parts look in your entire closed-loop system. Supply: Monolithic Energy Programs
The design engineer’s important curiosity is to review the habits of the converter underneath load adjustments. Contemplating a set enter voltage (VIN), the open-loop switch operate might be modeled underneath small perturbations produced within the obligation cycle to review the ability provide’s dynamic response.
The summarized open-loop system might be modeled with Equation 1: 
(1)
The place G is the current-sense achieve remodeled to voltage and GC(s) and GCI(s) are the switch features of the flyback converter when it comes to output voltage and magnetizing present response (respectively) underneath small perturbations within the obligation cycle. GαTS is the modeling of the ramp compensation to keep away from the double-pole oscillation at half of the switching frequency.
Flyback converter management design
There are numerous selections and tradeoffs concerned in designing the flyback converter’s management loop. The next sections of the article will clarify the design course of step-by-step. Determine 2 reveals the design movement.
Determine 2 The design movement highlights management loop creation step-by-step. Supply: Monolithic Energy Programs
Management loop design course of and calculations
Step 1: Design inputs
As soon as the converter’s important parameters have been designed in keeping with the related specs, it’s time to outline the parameters as inputs for the management loop design. These parameters embrace the enter and output voltages (VIN and VOUT, respectively), operation mode, switching frequency (fSW), obligation cycle, magnetizing inductance (LM), turns ratio (NP:NS), shunt resistor (RSHUNT), and output capacitance (COUT). Desk 1 reveals a abstract of the design inputs for the circuit mentioned on this article.
Desk 1 Here’s a abstract of design inputs required for creating management loop. Supply: Monolithic Energy Programs
To design a flyback converter compensator, it’s essential to first acquire all important parts that make the converter. Right here, HF500-40 flyback regulator is used to display design of a compensator utilizing optocoupler suggestions. This system is a fixed-frequency, current-mode regulator with built-in slope compensation. As a result of the converter works in steady conduction mode (CCM) at low line enter, a double-pole oscillation at half of the switching frequency is produced; built-in slope compensation dampens this oscillation, making its impact virtually null.
Step 2: Calculate parameters of the open-loop switch operate
It’s very important to calculate the parameters of the open-loop switch operate and calculate the values for all the compensator’s parameters that may optimize the converter on the dynamic habits degree.
The open-loop switch operate of the height present management flyback converter (additionally together with the compensation ramp issue) might be estimated with Equation 2:
(2)
The place D’ is outlined by the proportion of time that the secondary diode (or synchronous FET) is energetic throughout a switching cycle.
The fundamental canonical mannequin might be outlined with Equation 3: 
(3)
Observe that the equal collection resistance (ESR) impact on the output capacitors has been included within the switch operate, because it’s probably the most vital parasitic impact.
Through the use of Equation 2 and Equation 3, it’s doable to calculate the very important parameters.
The resonant frequency (fO) might be calculated with Equation 4:
(4)
After inputting the related values, fO might be calculated with Equation 5: 
(5)
The appropriate-half-plane zero (fRHP) might be estimated with Equation 6: 
(6)
The q-factor (Q) might be calculated with Equation 7: 
(7)
After inputting the related values, Q might be estimated with Equation 8: 
(8)
The DC achieve (Okay) might be calculated with Equation 9: 
(9)
After inputting the related values, Okay might be estimated with Equation 10: 
(10)
The high-frequency zero (fHF) might be calculated with Equation 11:
(11)
It’s necessary to notice that with present mode management, it’s frequent to acquire values nicely beneath 0.5 for Q. With this in thoughts, the results of the second-degree polynomial within the denominator of the switch operate finally ends up giving two actual and unfavorable poles. That is completely different from voltage-control mode or when there’s a very giant compensation ramp, which ends up in two advanced conjugate poles.
The 2 actual and unfavorable poles might be estimated with Equation 12: 
(12)
The brand new open-loop switch operate might be calculated with Equation 13: 
(13)
The cutoff frequency (fC) might be estimated with Equation 14: 
(14)
The next sections will clarify how the frequency compensator design achieves energy provide stability and wonderful efficiency.
Step 3: Frequency compensator design
As soon as the open-loop switch operate is modeled, it’s essential to design the frequency compensator such that it achieves one of the best efficiency doable. As a result of the frequency response of the above switch operate has two separate poles—one at a low frequency and one at a excessive frequency—a easy Kind II compensator might be designed. This compensator doesn’t want a further zero, which isn’t the case in voltage-control mode as a result of there’s a double pole that produces a resonance.
To reduce the steady-state error, it’s essential to design an inverted-zero (or a pole on the origin) as a result of it produces increased good points at low frequencies. To make sure that the system’s stability shouldn’t be impacted, the frequency should be at the least 10 instances decrease than the primary pole, calculated with Equation 15:
(15)
As a result of ESR parasitic impact at excessive frequencies, it’s essential to design a high-frequency pole to compensate for and take away this impact. The pole might be estimated with Equation 16:
(16)
However, it’s frequent to change the cutoff frequency to attain a better or decrease bandwidth and produce quicker or slower dynamic responses, respectively. As soon as the cutoff frequency is chosen (on this case, fC is elevated as much as 6.5 kHz, or 10% of fSW), the compensator’s middle-frequency achieve might be calculated with Equation 17: 
(17)
As soon as the compensator has been designed throughout the frequency vary, calculate the values of the passive parts.
Step 4: Design the compensator’s passive parts
The most typical Kind II compensator used for stabilization in present management mode flyback converters with cross-regulation is made up of an optocoupler suggestions (Determine 3).
Determine 3 Kind-II compensator is made up with optocoupler suggestions. Supply: Monolithic Energy Programs
The compensator switch operate based mostly on optocoupler suggestions might be estimated with Equation 18: 
(18)
The center-frequency achieve is fashioned in two levels: the optocoupler achieve and the adjustable voltage reference compensator achieve, calculated with Equation 19:
(19)
It’s necessary to calculate the utmost resistance to accurately bias the optocoupler. This resistance might be estimated with Equation 20:
(20)
The parameters essential to calculate RD might be discovered within the optocoupler and the adjustable voltage reference datasheets. Desk 2 reveals the everyday values for these parameters from the optocoupler.
Desk 2 Listed below are the principle optocoupler parameters. Supply: Monolithic Energy Programs
Desk 3 reveals the everyday values for these parameters from the adjustable voltage reference.
Desk 3 The above knowledge reveals adjustable voltage reference parameters. Supply: Monolithic Energy Programs
As soon as the above parameters have been obtained, RD might be calculated with Equation 21: 
(21)
As soon as the worth of R3 is obtained (on this case, R3 is inside to the HF500-40 controller, with a minimal worth of 12 kΩ), in addition to the values for R1, R2, and RD (the place RD = 2 kΩ), RF might be estimated with Equation 22:
(22)
The place GCOMP is the compensator’s center frequency achieve, calculated with Equation (17). GCOMP is used to regulate the ability provide’s bandwidth.
As a result of the inverted zero and high-frequency pole had been already calculated, CF and CFB might be calculated with Equation 23 and Equation 24, respectively. 
(23) 
(24)
As soon as the open-loop system and compensator have been designed, the loop achieve switch operate might be estimated with Equation 25: 
(25)
Equation 25 relies on Equation 13 and Equation 18.
It’s necessary to calculate the section and achieve margins to make sure the steadiness of energy provide.
The section margin might be calculated with Equation 26: 
(26)
After inputting the related values, the section margin might be calculated with Equation 27: 
(27)
If the section margin exceeds 50°, it’s an necessary parameter essential to adjust to sure requirements.
On the identical time, the achieve margin might be approximated with Equation 28: 
(28)
Equation 29 is derived from Equation 25 on the specified frequency:
(29)
On this situation, the achieve margin is beneath -10dB, which is one other necessary parameter to contemplate, significantly concerning compliance with regulation specs. If the result’s near 0dB, some iteration is important to lower the worth; in any other case, the efficiency is suboptimal. This iteration should begin by reducing the worth of the cutoff frequency.
This whole switch operate gives stability to the ability provide and one of the best efficiency made doable by:
- Minimizing steady-state error
- Minimizing ESR parasitic impact
- Rising bandwidth of energy provide as much as 6.5 kHz
Ultimate design
After calculating all of the passive element values for the suggestions loop compensator and figuring out the converter’s important parameters, your entire flyback might be designed utilizing the flyback regulator. Determine 4 reveals the circuit’s closing design utilizing all calculated parameters.
Determine 4 Right here is how the ultimate design circuit schematic appears to be like like. Supply: Monolithic Energy Programs
Determine 5 reveals the bode plot of the whole loop achieve frequency response.
Determine 5 Bode plot is proven for the whole loop achieve frequency response. Supply: Monolithic Energy Programs
Acquiring the flyback averaged mannequin through small-signal evaluation is a posh course of to most correct approximation of the converter’s switch features. As well as, the cross-regulation method includes secondary-side regulation by means of optocoupler suggestions and an adjustable voltage reference, which complicates calculations.
Nonetheless, by following the 4 steps defined on this article, a superb approximation might be obtained to enhance the ability provide’s efficiency, because the output with the heaviest load is straight regulated. Which means the output can react shortly to load adjustments.
Joan Mampel is software engineer at Monolithic Energy Programs (MPS).
Associated Content material
- Energy Ideas: Compensating Remoted Energy Provides
- Particulars on compensating voltage mode buck regulators
- Energy Ideas #139: Easy methods to simplify AC/DC flyback design with a self-biased converter
- Modeling and Loop Compensation Design of Switching Mode Energy Provides, Half 1
- Modeling and Loop Compensation Design of Switching Mode Energy Provides, Half 2
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