[Source: Gaogong LED 's " LED Good Products" magazine January issue / Long Zonghui]
It is recognized in the industry that the short board of LED power supply lies in electrolytic capacitors. The life of capacitors is basically equal to the life of power supply. Although many capacitor manufacturers have made a lot of efforts in technology, the life of capacitors is difficult to meet the requirements of 50,000 hours of power supply. match. In recent years, many manufacturers in the industry have proposed to solve the problem of the life of traditional electrolytic capacitors by including electroless capacitors or by setting capacitors as non-core devices.
Qi Zhaoxiong, technical manager of Shenzhen Sunlord Electronics Co., Ltd. (hereinafter referred to as “Shunluo Electronicsâ€, 002138.SZ), said that tantalum capacitors do not use electrolytes like ordinary electrolytic capacitors. They are solid capacitors, and they have almost no inductance at low frequencies. Long life, small size, high temperature resistance, high accuracy, excellent filtering performance, etc.
At present, tantalum capacitors produced by Sunlord Electronics are classified into manganese tantalum capacitors and polymer tantalum capacitors according to the cathode material. In terms of comprehensive performance, tantalum capacitors have obvious advantages in terms of capacitance, temperature, temperature stability, and lifetime. They are very suitable for LED-driven DC-DC converters, but their capacity will not be too high. Large, compared to electrolytic capacitors, there is no advantage in price.
Tantalum capacitors are highly reliable, and even after more than 10 years, its basic electrical performance has hardly changed. But we can't understand that tantalum capacitors are highly reliable and don't have to be derated when in use. In addition to the normal operating voltage in the circuit, surge voltage and AC ripple voltage can cause the tantalum capacitor to fail, so we must consider it at design time.
In addition, in the actual working environment, due to the heat generated by the circuit, the temperature of the tantalum capacitor working chamber may reach 50 ° C or more. Therefore, in actual use, the problem that the leakage current of the tantalum capacitor increases due to the temperature increase must also be considered.
Table 1 é’½ Capacitance Performance Comparison Table
At present, there are two types of chip tantalum capacitor circuits: low impedance circuits and circuits with resistance protection (also called high impedance circuits). In the low-impedance circuit, the main function of the chip tantalum capacitor is to filter or simultaneously have the energy storage discharge, but in the energy storage discharge circuit, because the actual parameters used by the circuit, such as the charge and discharge power are different, therefore, the chip type Capacitor performance choices must also have different priorities.
Taking the DC-DC circuit as an example, the main function of the capacitor is to use the polarity of the electrolytic capacitor to filter out the residual AC signal in the DC signal input from the front stage as much as possible, or to make it smaller, so as to avoid DC after input. Excessive AC component in the signal causes the power device in the circuit to generate heat and reduce AC noise and signal interference in the circuit.
At the same time, the above circuit must provide a certain frequency of power output to the subsequent circuit, therefore, the circuit loop resistance must be very low, generally must be less than 1 ohm. According to the basic electrical signal characteristics of such circuits, at the moment of switching, the input voltage and current will produce a spike with a duration of less than 1 microsecond, and the peak voltage will reach a stable working voltage 2-3 in an instant. The peak circuit will be more than ten times larger than the steady-state operating current.
Therefore, the higher the impact strength of the spike signal in a short period of time, the tantalum capacitor used in such a circuit must be greatly derated, and the use voltage cannot exceed 1/3 of the rated voltage is a basic rule, because the reason is to avoid The spike will instantaneously exceed the rated voltage. If the output power is reduced, it is also possible to use a design with a rated voltage greater than twice the operating voltage. Otherwise, it must be derated significantly to ensure sufficient reliability.
Unfinished:
For more information, please refer to the January issue of Gaogong LED 's " LED Good Products" magazine.
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