WO2012104062A1

WO2012104062A1 - Driver circuit for organic led

Info

Publication number: WO2012104062A1
Application number: PCT/EP2012/000414
Authority: WO
Inventors: Jamie Kelly
Original assignee: Tridonic Uk Ltd.
Priority date: 2011-01-31
Filing date: 2012-01-31
Publication date: 2012-08-09
Also published as: GB2504220A; GB201313327D0; GB2504220B

Abstract

Driver circuit for organic LED (LED) using a flyback converter with a primary switch (Q1) and a power transformer (T2), whereby the primary switch (Q1) is clocked with high frequency and the power transformer (T2) is transferring the energy to a secondary side, whereby on the secondary side a active rectifier switch (Q2) is applied for the supply of the organic LED (LED), characterized by an inductive driving element (T1, L1), which is not magnetically coupled to the power transformer (T2), where the inductive driving element (T1, L1) is using the current flow through the primary switch (Q1 ) to turn off the active rectifier switch (Q2).

Description

Driver circuit for organic LED

The present invention relates to a method for driving an organic LED. Technical area

In order to save energy, today energy saving lamps are used. With the introduction of efficient inorganic light emitting diodes (LED) as potential light sources there is also a demand for energy saving lamps using LED which have the ability to be dimmed. In recent time there are developments to establish and use organic LED (OLED) as a light source for lighting.

State of the art Various circuit arrangements have been proposed which shall add the functionality of energy efficiency to LED based light sources.

One example would be the circuit shown in Fig. 1 , showing a flyback converter used for driving an inorganic LED. This circuit may be powered from a DC voltage, which may be generated from a rectified AC voltage. The transformer secondary winding has an over-wind that drives the gate of the active rectifier switch Q2. As a result, when current flows into the load, the gate of active rectifier switch Q2 is high causing the active rectifier switch Q2 to conduct and short its internal diode. This gives a low conduction drop. When the voltage reverses, active rectifier switch Q2 is held off and so active rectifier switch Q2 blocks the voltage and stops current flowing back to the transformer from the buffer capacitor C1.

The problem is the transition from active rectifier switch Q2 conducting to active rectifier switch Q2 not conducting. When active rectifier switch Q2 is conducting, it conducts in both directions and connects the transformer secondary across the buffer capacitor C1. To switch the active rectifier switch Q2 off requires the voltage across the transformer to change, but this cannot happen while active rectifier switch Q2 remains on. In case of an organic LED as load this can cause problems as the forward voltage is low compared to a typical LED module made of inorganic LEDs but the current through the organic LED is higher.

SUMMARY OF THE INVENTION

The present invention seeks to provide a further switching arrangement allowing an efficient driving of an organic LED driver with low power losses. According to the present invention, there is driver circuit for organic LED using a flyback converter with a primary switch and a power transformer, whereby the primary switch is clocked with high frequency and the power transformer is transferring the energy to a secondary side, whereby on the secondary side a active rectifier switch is applied for the supply of the organic LED,

characterized by an inductive driving element, which is not magnetically coupled to the power transformer, where the inductive driving element is using the current flow through the primary switch to turn off the active rectifier switch. The inductive driving element may be formed by a drive transformer or by a drive choke. A gate switch may be coupled to the inductive driving element, where switching the gate switch on will switch the active rectifier switch off. The gate switch may be coupled to the inductive driving element. A hold-resistor may be connected to the active rectifier switch which holds the active rectifier switch off during the switch on time of through the primary switch and holds the active rectifier switch on during the switch off time of through the primary switch. The hold-resistor may be coupled to charge capacitor, where the charge capacitor may be coupled to the secondary winding of the power transformer through a diode ln one aspect the present invention concerns a method for the drive of an organic LED, where energy for supply of the organic LED is transferred to a secondary side of a power transformer, and whereby the rectification on the secondary side is performed at least partly by active rectification, whereby the active rectification is controlled by an inductive driving element, which is not magnetically coupled to the power transformer.

The inductive driving element may use the current flow through the primary switch to turn on a gate switch and thereby turning off the active rectifier switch.

BRIEF DESCRIPTION OF THE DRAWINGS

As an aid to understanding the invention, a preferred embodiment thereof will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:

Figure 1 shows a circuit arrangement known from the state of the art; and Figure 2 shows a first embodiment bf a OLED drive circuit arrangement of the invention; and

Figure 3 shows a second embodiment of a OLED drive circuit arrangement of the invention; and Figure 4 shows the typical waveforms when applying this invention with the a second embodiment; and

Figure 5 shows a third embodiment of a OLED drive circuit arrangement of the invention; and

Figure 6 shows the typical waveforms when applying this invention using the third embodiment DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In a preferred embodiment of the invention the circuit is a part of a OLED driver circuit, e.g. as a part of a OLED lamp.

A first solution is shown in fig. 2. With this solution it is possible to switch the active rectifier switch Q2 off with a separate signal.

Figure 2 shows a power transformer T2. The primary winding of the power transformer T2 is connected in series with a primary switch Q1. The secondary winding of the power transformer T2 is connected through the active rectifier switch Q2 to the buffer capacitor C1 and the organic LED (LED) as the load. When the primary switch Q1 is switched on the input voltage of the flyback converter will be across the primary winding of the power transformer T2. Due to the coupling of the power transformer T2 there will be a negative voltage on the secondary side of the power transformer T2. When the primary switch Q1 is switched off the current through the primary winding of the power

transformer T2 will be interrupted and due to the magnetization of the power transformer T2 there will be a positive voltage on the secondary side of the power transformer T2. When active rectifier switch Q2 is switched on the energy of the power transformer T2 will be transferred to the buffer capacitor C1 and the organic LED (LED).

Figure 2 shows a drive transformer T1 added in series with the primary of power transformer T2. As the primary switch Q1 turns on, a current pulse flows in drive transformer T1 , switching the gate switch Q3 on. This switches off the active rectifier switch Q2 and allows the voltage on power transformer T2 to reverse. Once the voltage is reversed, the over-wind on power transformer T2 holds active rectifier switch Q2 off.

The drive transformer T1 acts as a pulse transformer only initiating the switch off of the active rectifier switch Q2. The hold-resistor R1 , which is coupled to the power transformer T2, will hold the active rectifier switch Q2 off. During the flyback period (where primary switch Q1 is off) hold-resistor R1 provides by its coupling to the power transformer T2 the drive to switch the active rectifier switch Q2 on. As power transformer T2 is still magnetized during the flyback period the voltage of the secondary winding of the power transformer T2 is applied through the hold-resistor R1 to the gate of the active rectifier switch Q2 and thus the voltage at the gate of the active rectifier switch Q2 is high.

As the primary switch Q1 switches on, the active rectifier switch Q2 is still switched on and thus connecting the power transformer T2 across the buffer capacitor C1. Therefore, the power transformer T2 cannot switch the active rectifier switch Q2 off. The drive transformer T1 is separate and uses the current flow in the primary switch Q1 to switch the gate switch Q3 on, therefore switching the active rectifier switch Q2 off. Once the active rectifier switch Q2 is off, hold-resistor R1 will hold it off as due to the coupling of the power transformer T2 the potential on the gate of the active rectifier switch Q2 is pulled down.

As gate switch Q3 is coupled to a secondary winding of the drive transformer T1 a current flow in the primary switch Q1 and thus though the primary side of the drive transformer T1 will lead to current through the secondary winding of the drive transformer T1 and through the base and emitter path of gate switch Q3. Such current will switch on gate switch Q3. By switching on of gate switch Q3 the potential on the gate of the active rectifier switch Q2 will be pulled down and the active rectifier switch Q2 is switched off.

Thus a driver circuit for organic LED (LED) using a flyback converter with a primary switch Q1 and a power transformer T2 is shown, whereby the primary switch Q1 is clocked with high frequency and the power transformer T2 is transferring the energy to a secondary side. On the secondary side a active rectifier switch Q2 is applied for the supply of the organic LED (LED). A drive transformer T1 is used as an inductive driving element T1 , which is not magnetically coupled to the power transformer T2. The inductive driving element T1 is using the current flow through the primary switch Q1 to turn on a gate switch Q3 and thereby turning off the active rectifier switch Q2. The driver circuit for the organic LED (LED) may be powered from a DC voltage, which may be generated directly from a rectified AC voltage or which can be supplied by a AC-DC- converter or a DC-DC-converter. Such AC-DC- converter or a DC-DC-converter can control the DC voltage, which is supplied to the driver circuit for the organic LED (LED).

This approach requires an over-wind on the secondary of the power transformer T2.

The circuit of figure 3 shows an alternative that uses a charge pump circuit (using the charge capacitor C2 and the pump diode D4) to generate a supply of which the active rectifier switch Q2 can be driven from. As the primary switch Q1 is clocked the power transformer T2 is repeatedly magnetized and demagnetized. Due to its coupling to the secondary winding of the power transformer T2 through the pump diode D4 the charge capacitor C2 is repeatedly charged and thus the charge capacitor C2 is charged to a voltage level which can be used as a supply for the hold-resistor R1 for the next switching cycle.

During the flyback period, hold-resistor R1 is fed by the charge capacitor C2 and provides the drive (switch) to the active rectifier switch Q2 on. As the primary switch Q1 switches on, the active rectifier switch Q2 is still on and connecting the power transformer T2 across the buffer capacitor C1.

Therefore, the power transformer T2 cannot switch the active rectifier switch Q2 off.

The drive transformer T1 acts as a pulse transformer only initiating the switch off of the active rectifier switch Q2 with the help of the gate switch Q3. The hold-resistor R1 in connection with transistor Q4 will hold the active rectifier switch Q2 switched off. The drive transformer T1 is separate to the power transformer T2 and uses the current flow in the primary switch Q1 to switch the gate switch Q3 on, therefore switching the active rectifier switch Q2 off. Once the active rectifier switch Q2 is switched off, hold-resistor R1 will hold it off.

Figure 4 shows the gate drive waveform of the active rectifier switch Q2, which equals the collector voltage of the gate switch Q3 (according to the circuit shown in fig. 3).

It would be an advantage to remove the drive transformer T1. This is a component that crosses the isolation barrier. Figure 5 shows a circuit arrangement that may achieve this. Instead of the drive transformer T1 of the examples of fig. 2 and fig. 3 a drive choke L1 may be used.

Similar to the example of fig. 2 and fig. 3 the emitter of the gate switch Q3 is directly linked to one end of the secondary output of the power transformer T2. When the primary switch Q1 switches on, a negative going pulse will be transformed by the power transformer T2 and the negative going pulse which is applied to the emitter of the gate switch Q3 removes the charge from the gate of the active rectifier switch Q2 and the active rectifier switch Q2 will switch off.

Once the active rectifier switch Q2 is off, the voltage on the power transformer T2 can fully reverse, charging through the diode d4 the charge capacitor C4 (similar to the charge capacitor C2 of fig. 3). The gate of the active rectifier switch Q2 is held low by the current path through resistor R5, the collector base junction of transistor Q4 and hold-resistor R1. Resistor R5, transistor Q4 and hold-resistor R1 are supplied by the charge capacitor C4. When primary switch Q1 switches off and power transformer T2 flies back (the flyback period begins) and due to the voltage on the secondary side of the power transformer T2 current flows to the output and the load, diode D6 will carry current for a short period of time. This will produce some losses.

However, drive choke L1 which is connected in parallel to the diode D6 will soon takes over, and with the active rectifier switch Q2 switched on, the losses are reduced to a low level. The hold-resistor R1 is supplied by the charge capacitor C4. Thus again the hold-resistor R1 will hold the active rectifier switch Q2 switched on until a negative going pulse is applied to the emitter of the gate switch Q3.

A drive choke L1 is used as an inductive driving element, which is not magnetically coupled to the power transformer T2. Figure 6 shows the gate drive waveform of the active rectifier switch Q2, which equals the collector voltage of the gate switch Q3 (according to the circuit shown in fig. 5).

The primary switch Q1 may be a bipolar transistor or a MOSFET. The active rectifier switch Q2 may be a bipolar transistor or a MOSFET. The gate switch Q3 may be a bipolar transistor or a MOSFET. The transistor Q4 may be a bipolar transistor or a MOSFET.

Instead of a flyback converter as shown in the examples of the fig. 2 to 5, the driver circuit for the organic LED (LED) may also comprise a different type of an isolated switched converter, e,g, a resonant halfbridge converter with transformer isolation or a forward converter. The driver circuit for the organic LED (LED) may also comprise a non-isolated switched converter, e.g. a buck converter or a buck-boost converter. In case of the use of a non-isolated switched converter there would be a power inductance L2 (not shown) be used instead of the power transformer T2. The inductive driving element T1 would not be magnetically coupled to the power inductance L2. The invention covers as well a method for the drive of an organic LED (LED), where energy for supply of the organic LED (LED) is transferred to a

secondary side of a power transformer T2, and whereby the rectification on the secondary side is performed at least partly by active rectification, whereby the active rectification is controlled by an inductive driving element T1 , L1 , which is not magnetically coupled to the power transformer T2. The active rectification on the secondary side is done by an active rectifier switch Q2. Advantageously the inductive driving element T1 , L1 is using the current flow through the primary switch Q1 to turn on a gate switch Q3 and thereby turning off the active rectifier switch Q2.

Generally the invention covers also a lighting system comprising an organic LED and a driver circuit for the organic LED according to this invention. Reference list:

Q1 primary switch

Q2 active rectifier switch

Q3 gate switch

Q4 transistor

R1 hold-resistor

R5 resistor

T1 drive transformer

L1 drive choke

T2 power transformer

C1 buffer capacitor

C2 charge capacitor

C4 charge capacitor

D4 pump diode

D6 diode

LED inorganic (prior art) or organic LED as load (fig. 2 to 5)

Claims

Claims:

Driver circuit for organic LED (LED) using a flyback converter with a primary switch (Q1 ) and a power transformer (T2), whereby the primary switch (Q1 ) is clocked with high frequency and the power transformer (T2) is transferring the energy to a secondary side,

whereby on the secondary side a active rectifier switch (Q2) is applied for the supply of the organic LED (LED),

characterized by an inductive driving element (T1 , L1 ), which is not magnetically coupled to the power transformer (T2), where the inductive driving element (T1 , L1 ) is using the current flow through the primary switch (Q1 ) to turn off the active rectifier switch (Q2).

Driver circuit for organic LED (LED) of claim 1 , where the inductive driving element (T1 , L1 ) is formed by a drive transformer.

Driver circuit for organic LED (LED) of claim 1 , where the inductive driving element (T1 , L1 ) is formed by a drive choke.

Driver circuit for organic LED (LED) of claim 1 , 2 or 3, where a gate switch (Q3) is coupled to the inductive driving element (T1 , L1 ), where switching the gate switch (Q3) on will switch the active rectifier switch (Q2) off.

Driver circuit for organic LED (LED) of claim 1 , 2, 3 or 4, where the gate switch (Q3) is coupled to the inductive driving element (T1 , L1 ).

Driver circuit for organic LED (LED) of any one of claims 1 to 5, where a hold-resistor (R1 ) is connected to the active rectifier switch (Q2) which holds the active rectifier switch (Q2) off during the switch on time of through the primary switch (Q1 ) and holds the active rectifier switch (Q2) on during the switch off time of through the primary switch (Q1 ). Driver circuit for organic LED (LED) of any one of claims 1 to 6, where the hold-resistor (R1 ) is coupled to charge capacitor (C4, C2), where the charge capacitor (C4, C2) is coupled to the secondary winding of the power transformer (T2) through a diode (D4).

Method for the drive of an organic LED (LED), where energy for supply of the organic LED (LED) is transferred to a secondary side of a power transformer (T2), and whereby the rectification on the secondary side is performed at least partly by active rectification, whereby the active rectification is controlled by an inductive driving element (T1 , L1 ), which is not magnetically coupled to the power transformer (T2).

Method according to claim X, where the inductive driving element (T1 , L1 ) is using the current flow through the primary switch (Q1 ) to turn on a gate switch (Q3) and thereby turning off the active rectifier switch (02).

10. A lighting system power supply arrangement substantially as

hereinbefore described with reference to and/or substantially as illustrated in any one of or any combination of Figures 2 to 6 of the accompanying drawings.