TWI897541B - LED driver - Google Patents

Abstract

The present invention provides a light-emitting diode (LED) driver power supply comprising a transformer and a transistor connected between the transformer's primary winding and ground. The LED driver power supply is configured to: obtain first and second sampled voltages by sampling the voltage across the transformer's primary winding; obtain an output characteristic voltage representing the output voltage of the LED driver power supply by performing an operation on the first and second sampled voltages; and generate an overvoltage protection signal indicating whether overvoltage protection is required by comparing the output characteristic voltage with a reference voltage.

Description

LED driver

The present invention relates to the field of circuits, and more specifically to a light emitting diode (LED) driver.

An LED driver is a power converter that converts AC power into a specific voltage and/or current to drive LEDs. The LED driver chip is a crucial component of an LED driver, its primary function being to ensure a constant output current to the LEDs. However, if an LED opens or other conditions cause the output voltage of the LED driver to increase, the LED driver chip must take appropriate measures to ensure that the output voltage does not exceed the withstand voltage limit of the output capacitor to prevent damage.

According to an embodiment of the present invention, an LED driver power supply includes a transformer and a transistor connected between the transformer's primary winding and ground. The LED driver power supply is configured to: obtain first and second sampled voltages by sampling the voltage across the transformer's primary winding; obtain an output characteristic voltage representing the output voltage of the LED driver power supply by performing an operation on the first and second sampled voltages; and generate an overvoltage protection signal indicating whether overvoltage protection is required by comparing the output characteristic voltage with a reference voltage.

100, 300, 400: LED driver

C1: output capacitor

CMP: Comparator

D1: diode

Drain: Drain voltage

FB: Output feedback voltage

HV: High Voltage

Ipeaks: Peak current

Ls: Inductance

M1, MP1, MP2: transistors

Np/Ns, Nf/Ns: turns ratio

Np: Primary Winding

Ns: Secondary Winding

OP1, OP2: First operational amplifier

OVP: Overvoltage protection signal

R1, R2: resistors

ROVP: Overvoltage protection threshold resistor

T: Transformer

t0: time

Tdem: Demagnetization time

Tref: base demagnetization time

Vbase: Base voltage

VD: Drain voltage

VD_di, VH_di: sampling voltage

VH: Supply voltage

VIN: Input voltage

Vo, Vo1, Vo2: output voltage

Vo_effi: output characteristic voltage

Vref: reference voltage

The present invention can be better understood from the following description of specific embodiments of the present invention in conjunction with the drawings, wherein:

Figure 1 shows a schematic diagram of a traditional LED driver and its overvoltage protection solution.

Figure 2 shows the waveforms of the key node voltages in the LED driver shown in Figure 1.

Figure 3 shows a schematic diagram of another traditional LED driver and its overvoltage protection solution.

Figure 4 shows a schematic diagram of an LED driver power supply and its overvoltage protection solution according to an embodiment of the present invention.

FIG5 is a schematic diagram showing an example circuit implementation of the operation module shown in FIG4.

Figure 6 is a waveform diagram of the voltage at key nodes in the LED driver shown in Figure 4.

The features and exemplary embodiments of various aspects of the present invention are described in detail below. In the detailed description below, many specific details are set forth in order to provide a comprehensive understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention can be implemented without some of these specific details. The following description of the embodiments is intended only to provide a better understanding of the present invention by illustrating examples of the present invention. The present invention is in no way limited to any specific configuration and algorithm set forth below, but rather covers any modifications, substitutions, and improvements to the elements, components, and algorithms without departing from the spirit of the present invention. In the drawings and the following description, well-known structures and techniques are not shown in order to avoid unnecessary ambiguity in the present invention. In addition, it should be noted that the term "A and B are connected" used here can mean "A and B are directly connected" or "A and B are indirectly connected via one or more other components."

Figure 1 shows a schematic diagram of a traditional LED driver and its overvoltage protection scheme. As shown in Figure 1, in the LED driver 100, when the transistor M1 is in the on state, the diode D1 is in the off state, and the output capacitor C1 supplies power to the load; when the transistor M1 is in the off state, the diode D1 is in the on state, and the voltage on the primary winding of the transformer T is equal to the voltage on the secondary winding. The ratio between them is equal to the turns ratio Np/Ns of the primary winding and the secondary winding. The ratio between the voltage on the auxiliary winding of the transformer T and the voltage on the secondary winding is equal to the turns ratio Nf/Ns of the auxiliary winding and the secondary winding. Resistors R1 and R2 divide the voltage on the auxiliary winding of the transformer T to obtain the characteristic LED. The output feedback voltage FB of the driving power supply 100 is compared with the reference voltage Vref to generate the overvoltage protection signal OVP. If the output feedback voltage FB is lower than the reference voltage Vref, it indicates that the output voltage Vo is not high (for example, lower than the withstand voltage limit of the output capacitor C1), and the overvoltage protection signal OVP is low, indicating that overvoltage protection is not required. If the output feedback voltage FB is higher than the reference voltage Vref, it indicates that the output voltage Vo is too high (for example, higher than or equal to the withstand voltage limit of the output capacitor C1), and the overvoltage protection signal OVP is high, indicating that overvoltage protection is required. Here, in order to detect the output voltage Vo, an auxiliary winding needs to be added, which increases the cost and size of the LED driver power supply 100.

Figure 2 shows the waveforms of key node voltages in the LED driver shown in Figure 1. Drain represents the drain voltage of transistor M1, FB represents the output feedback voltage FB, and OVP represents the overvoltage protection signal OVP. Assume that the forward voltage drop of diode D1 is 0.3V and the voltage divider ratio of resistors R1 and R2 is k. When the LED driver 100 is operating normally and transistor M1 is off, the output voltage Vo is typically Vo1, the voltage on the auxiliary winding of transformer T is ( Vo 1+0.3)× Nf/Ns , and the output feedback voltage FB is k*( Vo 1+0.3)× Nf/Ns . At this time, the output feedback voltage FB is lower than the reference voltage Vref, and the overvoltage protection signal OVP is low, indicating that overvoltage protection is not required. At time t0, the output voltage Vo increases from Vo1 to Vo2, and the output feedback voltage FB follows the output voltage Vo and increases to k*( Vo 2+0.3)× Nf/Ns. At this time, the output feedback voltage FB is higher than the reference voltage Vref, and the overvoltage protection signal OVP is high, indicating that overvoltage protection is required.

Currently, LED drivers are increasingly meeting market demands for low cost and small size. Using the demagnetization time Tdem of the primary winding of transformer T to provide overvoltage protection for the output voltage Vo can meet these low-cost requirements. Demagnetization time Tdem is the time it takes from the moment transistor M1 switches from on to off until the current flowing through the primary winding of transformer T drops to zero.

FIG3 shows a schematic diagram of another traditional LED driver and its overvoltage protection solution. Combining Figures 1 and 3 , it can be seen that LED driver power supply 300 differs from LED driver power supply 100 in that the overvoltage protection signal OVP is generated by comparing the demagnetization time Tdem with a reference demagnetization time Tref. If the demagnetization time Tdem is greater than the reference demagnetization time Tref, the output voltage Vo is not high (for example, lower than the withstand voltage limit of the output capacitor C1), and the overvoltage protection signal OVP is low, indicating that overvoltage protection is not required. If the demagnetization time Tdem is less than the reference demagnetization time Tref, the output voltage Vo is too high (for example, higher than or equal to the withstand voltage limit of the output capacitor C1), and the overvoltage protection signal OVP is high, indicating that overvoltage protection is required. Compared to LED driver 100, LED driver 300 does not require the auxiliary winding of transformer T to detect the output voltage Vo, so its cost is lower.

In the LED driver 300 shown in FIG3 , the relationship between the output voltage Vo and the demagnetization time Tdem is as follows:

Where Ipeaks is the peak current flowing through the primary winding of transformer T, and Ls is the inductance of the secondary winding of transformer T. Equation (1) shows that the output voltage Vo is affected by multiple factors, including the demagnetization time Tdem, the inductance Ls, and the ambient temperature. Therefore, using the demagnetization time Tdem to detect the output voltage Vo has low detection accuracy.

In view of the above circumstances, an LED driver according to an embodiment of the present invention is proposed. Compared to LED driver 100, this device does not require the auxiliary winding of transformer T to detect the output voltage Vo, thereby achieving low cost. Furthermore, compared to LED driver 300, this device detects the output voltage Vo by sampling and calculating the voltage across the primary winding of transformer T, thereby achieving high accuracy.

FIG4 shows a schematic diagram of an LED driver power supply and its overvoltage protection scheme according to an embodiment of the present invention. As shown in FIG4 , the LED driver power supply 400 includes a transformer T and a transistor M1 connected between the primary winding of the transformer T and ground. The LED driver power supply 400 is configured to obtain a first and a second sampled voltage by sampling the voltages at both ends of the primary winding of the transformer T (i.e., the drain voltage VD of the transistor M1 and the supply voltage VH for the LED driver power supply 400). By calculating the first and second sampled voltages VD_di and VH_di, an output characteristic voltage Vo_effi representing the output voltage Vo of the LED driver 400 is obtained. Then, by comparing the output characteristic voltage Vo_effi with the reference voltage Vref, an overvoltage protection signal OVP is generated to indicate whether overvoltage protection is required.

As shown in Figure 4, in some embodiments, when the output characteristic voltage Vo_effi is lower than the reference voltage Vref, the overvoltage protection signal OVP is low, indicating that overvoltage protection is not required. When the output characteristic voltage Vo_effi is higher than the reference voltage Vref, the overvoltage protection signal OVP is high, indicating that overvoltage protection is required.

In some embodiments, the computing module in the LED driver 400 can obtain the output characteristic voltage Vo_effi through the following processing: based on the voltage difference between the first and second sampled voltages VD_di and VH_di, using a first resistor to generate an output characteristic current representing the output voltage Vo; using a current mirror to mirror the output characteristic current to generate an image characteristic current representing the output voltage Vo; and based on the mirrored characteristic current, using a second resistor to generate the output characteristic voltage Vo_effi.

Figure 5 is a schematic diagram of an example circuit implementation of the operational module shown in Figure 4. As shown in Figure 5, in some embodiments, first and second operational amplifiers OP1 and OP2 are used to provide first and second sampled voltages VD_di and VH_di to both ends of a first resistor (e.g., R1). An output characteristic current is generated by the first resistor based on the voltage difference between the first and second sampled voltages VD_di and VH_di. A current mirror composed of transistors MP1 and MP2 is used to mirror the output characteristic current to generate a mirrored characteristic current. The voltage generated by the mirrored characteristic current flowing through a second resistor (e.g., R2) is added to the base voltage Vbase to generate an output characteristic voltage Vo_effi. Specifically, the output characteristic voltage Vo_effi can be expressed as the following equation (where k is the replication ratio of the current mirror):

It should be understood that the errors introduced by operational amplifiers OP1 and OP2 can be eliminated by matching them, and by matching resistors R1 and R2. The errors introduced by the current mirror can be improved by adjusting its structure, and the accuracy of the output characteristic voltage Vo_effi can be further improved by introducing a base voltage Vbase. In this way, the overall operational accuracy of the output characteristic voltage Vo_effi can be controlled within ±5%.

Figure 6 shows waveforms of key node voltages in the LED driver 400 shown in Figure 4. Drain represents the drain voltage of transistor M1, OVP represents the overvoltage protection signal (OVP), and Vo_effi represents the output characteristic voltage (Vo_effi). Assuming the forward voltage drop of diode D1 is 0.3V, when LED driver 400 is operating normally and transistor M1 is off, the drain voltage of transistor M1 is ( Vo + 0.3) × Np/Ns. The output voltage Vo is normally Vo1. At this time, the voltage difference between the two ends of the primary winding of transformer T is small, the output characteristic voltage Vo_effi is lower than the reference voltage Vref, and the overvoltage protection signal OVP is low, indicating that overvoltage protection is not required. At time t0, the output voltage Vo increases from Vo1 to Vo2, causing the drain voltage of transistor M1 to increase, thereby causing the first sampling voltage VD_di to increase, making the output characteristic voltage Vo_effi higher than the reference voltage Vref. The overvoltage protection signal OVP is high, indicating that overvoltage protection is required.

The present invention may be implemented in other specific forms without departing from its spirit and essential characteristics. For example, the algorithms described in the specific embodiments may be modified without departing from the basic spirit of the invention. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, and the scope of the invention is to be defined by the appended claims rather than the foregoing description, and all changes coming within the meaning and range of equivalents of the claims are intended to be included within the scope of the invention.

400:LED driver

C1: output capacitor

CMP: Comparator

D1: diode

Drain: Drain voltage

HV: High Voltage

Ls: Inductance

M1: Transistor

Np: Primary Winding

Ns: Secondary Winding

OVP: Overvoltage protection signal

ROVP: Overvoltage protection threshold resistor

T: Transformer

VD_di, VH_di: sampling voltage

VIN: Input voltage

Vo: output voltage

Vo_effi: output characteristic voltage

Vref: reference voltage

Claims (6)

An LED driver power supply includes a transformer and a transistor connected between a primary winding of the transformer and ground. The LED driver power supply is configured to: obtain first and second sampled voltages by sampling the voltages at both ends of the primary winding of the transformer; obtain an output characteristic voltage representing the output voltage of the LED driver power supply by performing an operation on the first and second sampled voltages; and obtain an output characteristic voltage representing the output voltage of the LED driver power supply by comparing the output characteristic voltage with a reference voltage. Generate an overvoltage protection signal indicating whether overvoltage protection is required; generate an output characteristic current representing the output voltage of the LED driver power supply using a first resistor based on the voltage difference between the first and second sampled voltages; mirror the output characteristic current using a current mirror to generate an image characteristic current representing the output voltage of the LED driver power supply; and generate the output characteristic voltage using a second resistor based on the mirrored characteristic current. The LED driver power supply according to claim 1, wherein the voltages across the primary winding of the transformer are the drain voltage of the transistor and the supply voltage for the LED driver power supply, respectively. The LED driver power supply as described in claim 1 is further configured to provide the first and second sampled voltages to both ends of the first resistor using first and second operational amplifiers. The LED driver power supply according to claim 1 is further configured to generate the output characteristic voltage by adding the voltage generated by the mirror characteristic current flowing through the second resistor and the base voltage. The LED driver power supply of claim 1, wherein when the output characteristic voltage is higher than the reference voltage, the overvoltage protection signal is high, indicating that overvoltage protection is required. The LED driver power supply of claim 1, wherein when the output characteristic voltage is lower than the reference voltage, the overvoltage protection signal is at a low level, indicating that overvoltage protection is not required.