A Rapid Design Method for Monolithic Switching Power Supply

When designing a switching power supply, the first problem we face is how to choose a suitable single-chip switching power supply chip that can meet the requirements without wasting resources due to improper selection. However, this is not an easy task. One reason is that single-chip switching power supplies have now formed four major series and nearly 70 models. Even if different models use the same package, their output power is different; the second reason is that when selecting a chip, not only the designed output power PO must be known, but also the efficiency η of the switching power supply and the power loss PD of the chip must be determined in advance. The latter two characteristic parameters can only be measured when the switching power supply is designed and installed. They are unknown before design.

The following focuses on the method of quickly selecting chips using the relationship curve between the power loss (PD) and power efficiency (η) and output power (PO) of the TOPSwitch-II series of single-chip switching power supplies, which can satisfactorily solve the above problems. Before designing, the most suitable single-chip switching power supply model and power loss value can be found from the curve based on the expected output power and power efficiency value. This not only simplifies the design, but also provides a reference for selecting the heat sink.

η/％(Uimin=85V)

Chinese Library Classification Number: TN86 Document Identification Code: A Article Code: 0219?2713(2000)09?488?05

PO/W

Figure 1 Relationship between PD and η, PO when wide input range and output is 5V

Figure 2 Relationship between PD and η, PO when wide input range and output is 12V

Figure 3 Relationship between PD and η, PO when the input is fixed and the output is 5V

Provided a basis.

1TOPSwitch-II PD vs η, PO relationship curve

The AC input voltage of TOPSwitch-II series is divided into two types: wide range input (also called universal input) and fixed input (also called single voltage input). The AC input voltages of the two are Ui=85V～265V and 230V±15% respectively.

1.1 Relationship curve between PD, η, PO in wide input range

TOP221~TOP227 series single-chip switching power supply under wide input range (85V~265V) conditions, when UO=+5V or +12V, the relationship curves of PD, η, and PO are shown in Figure 1 and Figure 2 respectively. Here, it is assumed that the minimum AC input voltage Uimin=85V and the maximum

η/％(Uimin=85V)

η/％(Uimin=195V)

The AC input voltage Uimax = 265V. The horizontal axis in the figure represents the output power PO, and the vertical axis represents the power efficiency η. The 7 solid lines drawn correspond to the power efficiency of TOP221~TOP227, and the 15 dotted lines are the isovalue lines of chip power consumption (the same below).

1.2 Relationship curve between PD, η and PO at fixed input

Under the condition of fixed AC input (230V±15%), when UO=＋5V or ＋12V, the relationship curves of PD, η and PO of TOP221~TOP227 series are shown in Figure 3 and Figure 4 respectively. These two curve families are also applicable to 208V, 220V and 240V. Now assume that Uimin=195V, Uimax=265V.

2. How to correctly select TOPSwitch-II chip

The design procedure for quickly determining the TOPSwitch-II chip model using the above relationship curve is as follows:

(1) First determine which curve is applicable. For example, when Ui = 85V ~ 265V, UO = +5V, you should choose Figure 1. When Ui = 220V (i.e. 230V - 230V × 4.3%), UO = +12V, you can only choose Figure 4;

(2) Then find the output power point position (PO) to be designed on the horizontal axis;

(3) Move vertically upward from the output power point until you select the solid curve indicated by the appropriate chip. If it is not applicable, continue to search upward for another solid line;

(4) Then read the power consumption PD of the chip from the contour line (dashed line). Then the junction temperature (Tj) of the chip can be calculated to determine the size of the heat sink;

(5) Finally, it enters the circuit design stage, including high-frequency transformer design, selection of peripheral component parameters, etc.

The following will illustrate this through three typical design examples.

Example 1: Design a universal switching power supply with an output of 5V and 300W

Universal switching power supply means that the AC input voltage range is 85V to 265V. Because UO=+5V, we must check the curve shown in Figure 1. First, find the output power point PO=30W on the horizontal axis, then move vertically upward to intersect with the solid line of TOP224 at a point, find out η=71.2% of the point on the vertical axis, and finally find PD=2.5W from the contour line passing through this point. This shows that if TOP224 is selected, 30W power can be output, and the expected power efficiency is 71.2%, and the chip power consumption is 2.5W.

If you think that the efficiency index of η=71.2% is too low, you can continue to look up for the solid line of TOP225. Similarly, if you choose TOP225, you can also output 30W power, and the expected power efficiency will be increased to 75%, and the chip power consumption will be reduced to 1.7W.

Based on the obtained PD value, the heat sink design can be completed. This is because the estimation of the power consumption of the chip used before the design is completely reliable.

Example 2: Design a switching power supply with a fixed input of 230V ± 15% AC and a DC output of 12V, 30W.

Figure 4 Relationship between PD and η, PO when the input is fixed and the output is 12V

η/％(Uimin=195V)

Figure 5 Relationship between K and Uimin′ for wide range input

Figure 6 Relationship between K and Uimin′ when input is fixed

According to the known conditions, it can be found from Figure 4 that TOP223 is the best choice, at this time PO = 30W, η = 85.2%, PD = 0.8W.

Example 3: Calculation of TOPswitch-II junction temperature

The junction temperature here refers to the die temperature Tj. Assume that the thermal resistance from the junction to the device surface is RθA (which includes the thermal resistance Rθ1 from the TOPSwitch-II die to the case and the thermal resistance Rθ2 from the case to the heat sink) and the ambient temperature is TA. Then find the PD value from the relevant curve and use the following formula to calculate the junction temperature of the chip:

Tj=PD·RθA＋TA(1)

For example, the design power consumption of TOP225 is 1.7W, RθA=20℃/W, TA=40℃, and Tj=74℃ is obtained by substituting it into formula (1). When designing, it must be ensured that the chip junction temperature Tj is lower than 100℃ at the maximum ambient temperature TAM, so that the switching power supply can work normally for a long time.

3. Correct the equivalent output power and other parameters according to the output power ratio

3.1 Correction method

As mentioned above, the relationship curves between PD and η, PO all restrict the minimum AC input voltage. Figures 1 and 2 specify Uimin=85V, while Figures 3 and 4 specify Uimin=195V (i.e. 230V-230V×15%). If the minimum AC input voltage does not meet the above regulations, it will directly affect the correct selection of the chip. At this time, the input power PO′ corresponding to the actual minimum AC input voltage Uimin′ must be converted into the equivalent power PO when Uimin is the specified value before the above 4 figures can be used. The conversion factor is also called the output power ratio (PO′/PO) and is represented by K. In the two cases of wide range input and fixed input of TOPSwitch-II, the characteristic curves of K and U′min are shown as the solid lines in Figures 5 and 6 respectively. A few points need to be explained:

(1) The minimum values ​​of the rated AC input voltage Uimin in Figures 5 and 6 are 85V and 195V respectively. The horizontal axis in the figure only marks the voltage range of Ui at the low end.

(2) When Uimin′>Uimin, K>1, that is, PO′>PO, which indicates that the originally selected chip now has a larger available power. If necessary, a chip with a slightly lower output power can be selected. When Uimin′ (3) Assume that the primary voltage is UOR, and its typical value is 135V. However, when Uimin′<85V, limited by the duty cycle adjustment capability of TOPSwitch-II, UOR will reduce UOR′ according to the linear law. At this time, the conversion factor K="UOR′" /UOR<1. The dotted lines in Figures 5 and 6 represent the characteristic curves of UOR′/UOR and Uimin′, which can be used to correct the primary induction voltage value.

The working procedures for correcting the output power are summarized as follows:

(1) First, select the appropriate characteristic curve from Figure 5 or Figure 6, and then find the conversion factor K based on the known Uimin′ value.

(2) Convert PO′ to the equivalent power PO when Uimin is the specified value, and the formula is:

PO = PO′ / K (2)

(3) Finally, select the appropriate relationship curve from Figures 1 to 4, and find the appropriate chip model and η and PD parameter values ​​based on the PO value.

The following is a typical example to illustrate the correction method.

Example 4: Design a 12V, 35W universal switching power supply

Given Uimin=85V, assume Uimin′=90%×115V=103.5V. From Figure 5, we can find that K=1.15. Substitute PO′=35W and K=1.15 into formula (2) and calculate PO=30.4W. Based on the PO value, we can find from Figure 2 that the best choice should be the TOP224 chip, where η=81.6% and PD=2W.

If TOP223 is selected, η will drop to 73.5%, and PD will increase to 5 W, which is obviously not suitable. If TOP225 is selected, it will cause a waste of resources because it is more expensive than TOP224 and is suitable for outputting a higher power of 40W to 60W.

3.2 Correction and selection of relevant parameters

(1) Correction of primary inductance

When using the TOPSwitch-II series to design a switching power supply, the typical parameters of the high-frequency transformer and related components are shown in Table 1. These values ​​can be used as preliminary selection values. When Uimin′LP′=KLP（3）

From Table 1, we can see that when TOP224 is used, LP = 1475μH. When K = 1.15, LP′ = 1.15 × 1475 = 1696μH.

Table 2 Optocoupler parameters change with Uimin′

Minimum AC input voltage Uimin(V) 85 195

LED operating current IF (mA) 3.5 5.0

Emitter current IE of phototransistor (mA) 3.5 5.0

(2) Impact on other parameters

When the specified value of Uimin changes, the duty cycle of TOPSwitch-II also changes, which in turn affects the LED operating current IF and the phototransistor emitter current IE in the optocoupler. At this time,
IF and IE should be readjusted according to Table 2.

TOPSwitch-II has power supply parameter values ​​independent of Ui and PO, see Table 3. These parameters are generally not affected by changes in Uimin.

Table 3 Power supply parameter values ​​independent of Ui and PO

Typical values ​​of independent parameters

Switching frequency f(kHz) 100

Input protection circuit clamping voltage UB(V) 200

Forward voltage drop UF (V) of output stage Schottky rectifier diode 0.4

Initial bias voltage UFB(V) 16

(3) Selection of input filter capacitor

Parameters TOP221 TOP222 TOP223 TOP224 TOP225 TOP226 TOP227

High frequency transformer primary inductance LP (μH) 8650 4400 2200 1475 1100 880 740

High frequency transformer primary leakage inductance LPO (μH) 175 90 45 30 22 18 15

Resonant frequency fO of high frequency transformer when secondary is open (kHz) 400 450 500 550 600 650 700

Primary coil resistance RP (mΩ) 5000 1800 650 350 250 175 140

Secondary coil resistance RS (mΩ) 20 12 7 5 4 3.5 3

DC resistance RL1 of output filter inductor (mΩ) 40 32 25 20 16 13 10

Common mode choke DC resistance RL2 (mΩ) 400 370 333 300 267 233 200