Resulting from their versatility, ease of design, and low price, flyback converters have turn into one of the broadly used topologies in energy electronics. Its construction derives from one of many three fundamental topologies—particularly, buck-boost topology. Nonetheless, not like buck-boost converters, flyback topologies enable the voltage output to be electrically remoted from the enter energy provide. This characteristic is significant for industrial and shopper purposes.
Among the many completely different management strategies used to stabilize energy converters, probably the most broadly used management methodology is peak present mode, which constantly senses the first present to supply necessary safety for the facility provide.
Moreover, to acquire the next design efficiency, it’s widespread to control the converter with the output that has the very best load utilizing a method referred to as cross-regulation.
This text goals to indicate engineers easy methods to accurately design the management loop that stabilizes the flyback converter as a way to present optimum performance. This course of consists of minimizing the stationary error, rising/reducing the bandwidth as required, and rising the section/acquire margin as a lot as doable.
Closed-loop flyback converter block diagram
Earlier than making the mandatory calculations for the controller to stabilize the height present management mode flyback, it’s necessary to know the elements of all the closed-loop system: the converter averaged mannequin and the management loop (Determine 1).
The design engineer’s important curiosity is to review the conduct of the converter underneath load modifications. Contemplating a set enter voltage (VIN), the open-loop switch operate will be modeled underneath small perturbations produced within the responsibility cycle to review the facility provide’s dynamic response.
The summarized open-loop system will be modeled with Equation 1:
The place G is the current-sense acquire remodeled to voltage and GC(s) and GCI(s) are the switch capabilities of the flyback converter when it comes to output voltage and magnetizing present response (respectively) underneath small perturbations within the responsibility 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 a lot of 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 circulation.
Determine 2 The design circulation highlights management loop creation step-by-step. Supply: Monolithic Energy Techniques
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), responsibility 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 Techniques
To design a flyback converter compensator, it’s essential to first acquire all important elements that make the converter. Right here, HF500-40 flyback regulator is used to exhibit 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 nearly 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 conduct degree.
The open-loop switch operate of the height present management flyback converter (additionally together with the compensation ramp issue) will be estimated with Equation 2:
The place D’ is outlined by the proportion of time that the secondary diode (or synchronous FET) is lively throughout a switching cycle.
The fundamental canonical mannequin will be outlined with Equation 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 important parasitic impact.
By utilizing Equation 2 and Equation 3, it’s doable to calculate the very important parameters.
The resonant frequency (fO) will be calculated with Equation 4:
After inputting the related values, fO will be calculated with Equation 5:
The fitting-half-plane zero (fRHP) will be estimated with Equation 6:
The q-factor (Q) will be calculated with Equation 7:
After inputting the related values, Q will be estimated with Equation 8:
The DC acquire (Okay) will be calculated with Equation 9:
After inputting the related values, Okay will be estimated with Equation 10:
The high-frequency zero (fHF) will be calculated with Equation 11:
It’s necessary to notice that with present mode management, it’s widespread to acquire values effectively under 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 detrimental poles. That is completely different from voltage-control mode or when there’s a very massive compensation ramp, which leads to two complicated conjugate poles.
The 2 actual and detrimental poles will be estimated with Equation 12:
The brand new open-loop switch operate will be calculated with Equation 13:
The cutoff frequency (fC) will be estimated with Equation 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 the perfect 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 will 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 attenuate the steady-state error, it’s essential to design an inverted-zero (or a pole on the origin) as a result of it produces larger beneficial properties at low frequencies. To make sure that the system’s stability is just not impacted, the frequency have to be no less than 10 occasions decrease than the primary pole, calculated with Equation 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 will be estimated with Equation 16:
Alternatively, it’s widespread to switch the cutoff frequency to attain the next or decrease bandwidth and produce sooner 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 acquire will be calculated with Equation 17:
As soon as the compensator has been designed inside the frequency vary, calculate the values of the passive elements.
Step 4: Design the compensator’s passive elements
The commonest 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 Techniques
The compensator switch operate primarily based on optocoupler suggestions will be estimated with Equation 18:
The center-frequency acquire is shaped in two phases: the optocoupler acquire and the adjustable voltage reference compensator acquire, calculated with Equation 19:
It’s necessary to calculate the utmost resistance to accurately bias the optocoupler. This resistance will be estimated with Equation 20:
The parameters essential to calculate RD will 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 here are the principle optocoupler parameters. Supply: Monolithic Energy Techniques
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 Techniques
As soon as the above parameters have been obtained, RD will be calculated with Equation 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 will be estimated with Equation 22:
The place GCOMP is the compensator’s center frequency acquire, calculated with Equation (17). GCOMP is used to regulate the facility provide’s bandwidth.
As a result of the inverted zero and high-frequency pole have been already calculated, CF and CFB will be calculated with Equation 23 and Equation 24, respectively.
As soon as the open-loop system and compensator have been designed, the loop acquire switch operate will be estimated with Equation 25:
Equation 25 relies on Equation 13 and Equation 18.
It’s necessary to calculate the section and acquire margins to make sure the steadiness of energy provide.
The section margin will be calculated with Equation 26:
After inputting the related values, the section margin will be calculated with Equation 27:
If the section margin exceeds 50°, it’s an necessary parameter essential to adjust to sure requirements.
On the identical time, the acquire margin will be approximated with Equation 28:
Equation 29 is derived from Equation 25 on the specified frequency:
On this situation, the acquire margin is under -10dB, which is one other necessary parameter to think about, significantly relating to 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 supplies stability to the facility provide and the perfect efficiency made doable by:
Closing design
After calculating all of the passive part values for the suggestions loop compensator and figuring out the converter’s important parameters, all the flyback will be designed utilizing the flyback regulator. Determine 4 reveals the circuit’s remaining design utilizing all calculated parameters.
Determine 4 Right here is how the ultimate design circuit schematic seems to be like. Supply: Monolithic Energy Techniques
Determine 5 reveals the bode plot of the entire loop acquire frequency response.
Determine 5 Bode plot is proven for the entire loop acquire frequency response. Supply: Monolithic Energy Techniques
Acquiring the flyback averaged mannequin through small-signal evaluation is a fancy course of to most correct approximation of the converter’s switch capabilities. As well as, the cross-regulation approach entails 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 great approximation will be obtained to enhance the facility provide’s efficiency, because the output with the heaviest load is instantly regulated. Because of this the output can react rapidly to load modifications.
Joan Mampel is software engineer at Monolithic Energy Techniques (MPS).
Associated Content material
The put up Design a suggestions loop compensator for a flyback converter in 4 steps appeared first on EDN.
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