Design of multiple switching power supplies using high-frequency monolithic switching chips

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Design of multiple switching power supplies using high-frequency monolithic switching chips [Copy link]

1. Excellent performance opens up new options for portable devices

For many years, the selection of power control chips with relatively high performance and price for designing multiple switching regulated power supplies has always been a difficult problem for the manufacturing industry. This is because power control chips either have too many pins and complex debugging, or have too few pins and unsatisfactory functions.

The TOPSwitchlI single-chip switching power supply is a relatively new high-frequency switching power supply chip launched by PI (Power Integration) in the United States. It can integrate all the necessary high-voltage N-channel power MOS field-effect transistors, voltage-type PWM controllers, 100kHz high-frequency oscillators, high-voltage startup bias circuits, reference voltages, parallel bias regulators for loop compensation, error amplifiers, and fault protection function blocks for the switching power supply. It is a high-frequency switching power supply chip with few pins (only 3 wires) and powerful functions.

It can be widely used in instruments, notebook computers, VCD and DVD, battery chargers, power amplifiers and other fields. The switching power supply formed by it has the advantages of light weight, small size, high efficiency, wide voltage regulation range, etc. It has been widely used in electronic equipment in many fields such as electronics, control, and computers. For this reason, this article will introduce the design of multiple switching voltage regulators using the TOP222Y high-frequency single-chip switching power supply control chip as the core.

2. Multiple (5) switching power supply design scheme

2.1 The power supply composition diagram with TOP222Y high-frequency single-chip switching power supply control chip as the core is shown in Figure 1. TOP222Y is a DC/DC converter, and its chip pins 3, 2, and 1 are respectively connected to the high-frequency transformer input and primary, output secondary and ground, and output feedback .

2.2 The power supply circuit topology is single-ended flyback

The power supply circuit topology is single-ended flyback, which means that when the power switch MOSFET is turned on, the electric energy is stored in the primary coil of the high-frequency transformer; when the MOSFET is turned off, the electric energy is output to the secondary. Since the switching frequency is as high as 100KHz, the high-frequency transformer can quickly store and release energy, and continuous output can be obtained after high-frequency rectification and filtering.

2.3 Function of single-stage power filter

The 220V AC incoming line is connected to an electromagnetic filter (EMl). In order to reduce the volume and reduce the cost, the monolithic switching power supply generally adopts a simple single-stage filter. L1 is used to filter out common-mode interference, and C1 and C2 are used to filter out series-mode interference. The function of the power filter: on the one hand, it filters out the clutter voltage transmitted from the power grid and purifies the input power supply. On the other hand, it also prevents the oscillation voltage of the high-frequency switching power supply from entering the power grid and interfering with other electrical appliances.

2.4 Rectification and DC/DC Converter

After rectification and capacitor filtering, the AC power becomes a 308V DC voltage to supply the TOP222Y device. TOP222Y constitutes a DC/DC converter, which converts the input DC high voltage into a high-frequency pulse voltage with adjustable pulse width, and then performs half-wave rectification and filtering after the voltage is stepped down by a high-frequency transformer to become the required DC voltage output.

2.5 Transient Voltage Suppression Circuit

The blocking diode D6 and the transient voltage suppressor D5 form an absorption circuit to absorb the peak voltage generated by the transformer leakage inductance during the shutdown process of the power device. When the TOP222Y power tube is turned on, the voltage polarity of the primary transformer is positive at the upper end and negative at the lower end, so that D6 is turned off and the clamping circuit does not work. At the moment when the MOSFET is turned off, the primary transformer becomes positive at the lower end and negative at the upper end. At this time, D6 is turned on and the peak voltage is absorbed by D5.

2.6 About high-frequency transformer and feedback voltage regulator circuit

The secondary of the high-frequency transformer has 5 windings, among which the 13.2V/300mA winding V1 is the main winding to control the pulse width of the TOP222Y device, that is, this group of output voltages is PWM regulated, and the sampling feedback work is completed by the parallel programmable voltage regulator TL431 and the optocoupler PC817 and the voltage divider resistors R4 and R5. When the output voltage increases, the sampling voltage obtained after the voltage division by R4 and R5 is compared with the bandgap reference voltage in TL431, so that the cathode potential of TL431 decreases, the working current If flowing through the photodiode increases, and then the control terminal current Ic increases through the optocoupler PC817, the output duty cycle of TOP222Y decreases, and the voltage decreases. The purpose of voltage regulation is achieved.

Resistor R3 is the minimum load of V1 output, which is used to improve the voltage regulation rate under light load. When the output voltage is low, the role of R3 is to provide a current bias path for 431. In order to avoid overshoot of the output voltage when the power is just turned on, a soft start capacitor C12 is connected in parallel between the cathode and anode of TL431. Its role is analyzed as follows: When the power is just turned on, the voltage drop across C12 cannot change suddenly, making VKA=0, and TL431 does not work. As the output voltage of the rectifier filter gradually increases, the current on the optocoupler diode (LED) charges C12 through R2, causing the voltage on C12 to continue to increase, and TL431 gradually enters normal working state. The output voltage rises slowly during the delay time and finally reaches a stable value of 13.2V.

2.7 Determination of sampling and feedback resistance

How to determine the values of R2, R3, R4 and R5. First, we need to understand the control characteristics of the TOP tube. From the technical manual of TOPSwicth, we know that the current Ic flowing into the control pin C is inversely proportional to the duty cycle D, as shown in Figure 2. It can be seen that the current of Ic should be between 2-6mA, and PWM will change linearly, so the current Ice of the PC817 transistor should also change in this range. Ice is controlled by the diode current If. The value of R1 must ensure that the TOP control end obtains the required current. Assuming that PC817 is used, its CTR=Ic/IF=0.8-1.6. From the technical parameters of TL437, when Vka changes from 2.5V to 36V, the cathode operating current IKA can change in a wide range from 1mA to 100mA. When the photocoupler CTR takes the lower limit of 0.8, the maximum current flowing through the photodiode is IFMAX-=6/0.8=7.5mA, and the cathode voltage of TL431 VkA=Vo-VF-(IFMAX×R2)>2.5V, where VF is the forward voltage drop of the photocoupler diode. The typical value of VF is 1.2V.

VkA=13.2-1.2-7.5×R2>2.5V

R2<1.3k (take R2=250Ω)

431 requires at least 1mA of operating current, that is, when the current of R2 is close to zero, 431 must also be guaranteed to have 1mA, so R3<=1.2V/1mA=1.2K (R3=510Ω is sufficient).

The value of R5 is not arbitrary. Two factors should be considered:

* 431 reference input current , generally this current is about 2μA, in order to avoid this end current affecting the voltage divider ratio and avoid the influence of noise, generally take the current flowing through resistor R5 to be more than 100 times the reference current, so this resistor should be smaller than

2.5V/200μA=12.5K．

* Standby power consumption requirement , if there is such a requirement, try to take the larger value while satisfying R5≤12.5KΩ. Take R5=10KΩ.

After determining the above relationships, the value of R4 resistor is easy to determine. According to the performance of TL431, R4, R5, Vo, and Vr have a fixed relationship: Vo = (1 + R4/R5) Vr (Vr = 2.5V), from which R4 = 43.kΩ can be calculated.

3. Design of main parameters of switching power supply circuit

Here we only introduce the calculation of main parameters such as the sum of output power of each group, output DC voltage, maximum duty cycle, primary current effective value and peak value, primary winding inductance value and primary pole winding turns.

3.1 The total output power of this power supply is the sum of the output power of each group:

PO=13.2×0.3+13.2×0.2+28×0.05+2×13.2×0.1+12×0.006=10.71W (the feedback winding power is 12×0.006)

If the total efficiency of the power supply is 80%, the total power input of the power supply should be:

Pi=PO/80％=10.7I/0.8=13.4W

Under a wide range of input voltage conditions, the maximum output power of TOP222Y is 15W, which can meet the requirements of this circuit.

3.2 Determine the minimum DC voltage and maximum DC voltage based on the input AC voltage

Assuming that the AC input voltage range is 85V-265V, the input rectifier bridge response time is tc=3mS, and the input filter capacitor C3 is 22uf, then for a wide range of voltage input, the input capacitor is selected in (2-3)PO units of μF, that is,

For wide range voltage input, the input capacitor is selected in μF (2-3)Po and is selected according to the proportionality factor (2~3) μF/W.

When the input capacitance is 33μF (recommended value), VMIN = 94V.

3.3 Determine the maximum duty cycle

The maximum duty cycle of the flyback power supply occurs at the lowest input voltage and maximum output power. According to the magnetic balance of the transformer in steady state, the following formula can be obtained:

If VOR is set to 100V, TOP drain-source voltage UDS = 10V, then DMAX = 0.6

The selection of flyback voltage VOR is not arbitrary. For wide range voltage input, it is generally 135V, and for multi-channel power output, it is generally 100V.

3.4 Calculate the effective value and peak value of the primary current

The primary working modes of the single-ended flyback converter are divided into two types: continuous mode and continuous mode. The primary winding current waveform is shown in Figure 3.

KRP is the current ripple coefficient. The value of KRP can be used to quantitatively describe the working mode of the switching power supply. When 0.4<KRP<1.0, it is in continuous mode, and when KRP=1, it is in discontinuous mode. A smaller KRP value means a more continuous working mode and a relatively large primary inductance, and the peak value and effective value of the primary current are smaller, so a smaller power TOPSwitch chip can be used.

Assuming that at the maximum duty cycle, when the switch is turned on, the primary current is Ip1, and when the switch is turned off, the primary current rises to Ip2. If Ip1 is 0, it means that the converter works in discontinuous mode, otherwise it works in continuous mode. According to the law of energy conservation, the following formula is obtained: 1/2·(Ipl+Ip2)*DMAX*VMIN=Pi

In order to improve efficiency, reduce power loss and reduce skin effect, we adopt continuous working mode: we set Ip2=2Ip2 so that we can calculate the primary peak current Ip2 of the converter:

(0.5 Ip2+ Ip2)×0.6×77=2×13.4 Ip2=0.387A

TOP222Y minimum limiting current IL1MIN = 0.45A, maximum limiting current

Value IL1MAX = 0.55A

The primary peak current Ip2 must satisfy:

Primary winding pulsating current

The ratio of the primary winding pulsating current Ip2 to the primary winding peak current Ip2

Primary winding effective current

3.5 Determine the primary winding inductance

Primary winding inductance:

3.6 Determine the number of turns of the primary pole winding

Select E122 core as the core selection basis (generally select the maximum magnetic flux density Bm = 0.2T-0.3T. The core below 0.2T is not fully utilized, and the ferrite material above 0.3 may be saturated). Bm is 0.25T (Tesla)

The number of turns of secondary V2, V4, V5 windings is N2=N4=N5=N1=11 turns

4. Conclusion

Since the TOP222Y high-frequency single-chip switching power supply control chip has few pins, the performance debugging of the multiple switching power supplies is convenient and simple, with low failure rate and high reliability.

As for the calculation of high-frequency transformers, there is no single answer. A large number of interrelated design variables need to be considered in the calculation process. Different variable values will result in some differences in the design results. Sometimes the theoretically calculated value will be different from the actual value, and further adjustments are required to meet the actual requirements.