# Sharing 9 circuit designs switching power supply, circuit diagram, circuit board, application notes

2023-09-19Archive

Sample application (1)

A simple 3-step lead-acid battery charger control scheme:

A simple 3-step lead-acid battery charger control scheme:

This board file is a simple 12V/4A three-stage charger designed in same way as above (no relay circuit).

Example Application (2)

Simple single TL431 current limiting and constant voltage control method:

When current increases, potential of TL431-1 is too high to play function of current current, since presence of resistor R3 compensates for output voltage. Therefore, functions of current limiting and voltage stabilization can basically be integrated, resulting in a relative cost advantage.

Sample application (3)

Scheme for starting a low-voltage xenon lamp:

This circuit is a half-bridge circuit that limits output power and uses a capacitor to limit current. (Adjust VR2 to get different starting voltages, and adjust VR1 to get different output currents to match different low voltage xenon lamps.)

Two windings are brought out, first one is main winding which can provide 27V30A, second one can provide 140V starting voltage, and starter current is limited to <0.5A through a capacitor connected in series in front of rectifying diode. At start, output voltage is based on feedback voltage of auxiliary winding. The open-circuit voltage of starting winding is limited to approximately 140V. After starting xenon lamp, voltage is forcibly reduced synchronously to about 27V of main winding voltage, since output power is limited by current sampling of input transformer, so 27V voltage will not be raised.

Due to current limiting method of series capacitors, circuit must operate at a fixed frequency to achieve synchronous start, and input voltage range cannot be too large. Generally, a change within 5% will not affect normal operation of xenon lamp.

A feature of this scheme is an effective solution to problem of synchronous launch, and achieving natural synchronization is more reliable than software control.

The peculiarity of starting a xenon lamp is that it must be fully synchronized, and it cannot be started at low voltage. But once current is started, voltage must be reduced to 24-28V at same time as current rises. If it is too large, there is a danger of lamp tube exploding. If current is below 25A, it will come out. However, it cannot be restarted immediately after shutdown. The application of this method can meet requirements effectively and at low cost.

Example Application (4)

Transformer isolation control circuit with ideal waveform:

Application example of a drive with ideal waveform transformer isolation:

Sample application (5)

Reference recommendations for design of a flyback switching power supply for a small transformer. This case is full voltage input of EC-2828 transformer, and output power is 60W.

EC-2828 full voltage input transformer, 60W output power.

For transformer design with small magnetic core: main problem is that magnetic core area Ae is too small, which will cause number of primary turns to be too large. The operating frequency can be increased accordingly, in which case operating frequency is 70-75 kHz. Due to large number of turns, connection of primary and secondary windings will be more advantageous. Therefore, at moment of a short circuit, voltage of VCC winding will reach a relatively high level. In circuit diagram of this case, there is an overvoltage protection function SCR. This process is achievable because short circuit of secondary winding connects to VCC winding to lower its voltage to point where IC cannot start.

To achieve above characteristics: diameter of VCC winding must be small. Personally, I usually take it below 0.17mm. If it is less than 0.12mm, it is easy to be broken. This small wire diameter does not save copper, but copper wire impedance can be used to replace function many designers use to connect low resistance resistors in series with VCC rectifier diode, and this uses coil impedance itself to suppress AC. This is more efficient in this case, and it can prevent instantaneous impact from damaging function of downstream circuit.

The primary and secondary main windings should be nearest adjacent windings in order for connection to be more advantageous.

The interference generated by switching power supply is greatest when MOSFET-D terminal is operating (it is same terminal where RCD absorbing terminal is connected to transformer). When winding transformer, it is recommended to wind it on first winding of transformer and use it as a starting point. Let it hide in innermost layer of transformer so that shielding of copper wire of rear winding will better suppress interference.

When calculating number of turns of VCC winding, try to multiply minimum operating voltage of IC by 1.1 times as error value, not considering voltage drop on copper wire, because current before starting is very small, so this resistance has little effect, almost negligible. Before starting circuit, due to charging of start resistor at high voltage end, voltage across capacitor at VCC can charge up to IC's start voltage. circuit, VCC cannot be started because coil voltage setpoint is low. The circuit also won't start and usually behaves like a hiccup.

Why should I choose a value according to lower limit of operating voltage of chip? Since our secondary winding is wound next to primary winding, coupling effect will be relatively better. The short circuit test we do is also a secondary output short circuit, because coupling effect is good when secondary short circuit, VCCdrops quickly after a short burst, and circuit will be best protected when it drops before IC is turned off. Voltage. It should be noted that this voltage must be more than 1V above saturation conductance of MOSFET to avoid underexcitation.

It is also useful to reduce power consumption of chip itself. It is impossible to check whether this can increase life of chip, but stability should be higher.

Example application (6)

Relatively stable solution for flyback double output:

Dual flyback output circuit with relatively stable output:

This type of circuit is commonly used in low-power power supplies. To provide better cross regulation of two windings. We need to pay attention to some issues.

In this example, we typically set sample feedback terminal to 5V. If dual-channel sample cross-regulation speed can be worse, it may not even depend on idle and independent load. This method solves this problem. This method is less suitable for applications where two sets of voltages are widely separated. This will take an extra foot of transformer.

The power supply of feedback optocoupler is powered by 12V, and sample point is best placed in front of post-stage filter inductor. Because fluctuation in front of filter inductor more quickly reflects input PWM modulation state, even if opening degree of TL431 is determined, because fluctuation of 12V may cause current fed back from optocoupler to have a small difference, in case of a certain data feedback loop In circumstances, selection of sampling point of optocoupler power supply is more favorable for controlling balance of dynamic response and adjustment speed.

The 12V winding should be closer to primary winding. This more effectively ensures that 12V voltage change ratio is smaller because we get feedback and sampling from 5V side, so it is difficult to control 12V winding. Combining them will allow better control over balance of two windings. Although it cannot be absolutely good, it does have a certain referential value, relatively speaking.

The model mentioned on previous page can basically control error within +/- 5%, which is within allowable range. It is recommended that friends who like doing it can try it.

Sample application (7)

Positive and negative output supply voltage short circuit protection control circuit applied to power amplifier:

Description: Power amplifier power supply with dual positive and negative output voltage protection

1) Undervoltage short circuit protection circuit consisting of transistor Q1

When positive voltage is short-circuited, voltage drops below zener diode and is added to Q1 to drive voltage divider resistor to make Q1 conductive, and then a protection signal can be sent.

2) Q2 is an undervoltage short-circuit protection

When negative voltage is short-circuited, voltage rises to a zener diode connected in series with Q2's base, and when Q2 turns off, voltage signal at collector of Q2 passes through D2 to send a protection signal.

3) Q3 - protection indicator light control circuit

In practical application, this circuit must delay the positive and negative VCC power supply process from power on to normal power on, otherwise there will be a protective signal when power is turned on, resulting in power failure. -fine. If it is necessary to block it, this can be done by driving a silicon controlled rectifier blocking circuit consisting of a triode with a protective signal output.

Amp power supply circuit board with dual positive and negative output voltage protection:

Sample application (8)

Using LM358 to implement PWM control for LED brightness, output terminal current limiting, and voltage regulation:

In this application, PWM signal is directly added to current sampling signal, and overcurrent protection signal time is modulated by adjusting PWM width to adjust current limiting function.

It should be noted that PWM needs an inverting input, so lower duty cycle, more current supplied to LED. The longer duty cycle, lower LED current.

Sample application (9)

50W LED driver circuit with PFC:

50W Power Factor Corrected LED Driver Board: