Published:2011/8/4 21:24:00 Author:Phyllis From:SeekIC
By Ton Giesberts
Suppressing inductive spikes
Due to the physical dimensions of the components, parasitic self-inductance and overshoots will always be present. The consequences of this, particularly the inductive spikes (backemf) from the inductors in the output filters, can be partially suppressed by using Schottky diodes and ultra-fast-recovery diodes as clamping diodes. This job is performed by D3, D4, Do, D7, D10, Dll, D13 and D14. Diodes in DO-15 packages (MUR120) are fitted to the circuit board for D3, D4, D10 and Dll. Diodes D6, D7, D13 and D14, which are SMD components in DO-214AA packages (MURS120T3), are fitted on the solder side underneath the IC (Figure 2). Both types of diodes are rated at 1 A / 200 V with a recovery time of only 25 ns.
MOSFET drive circuits
In the printed circuit board layout, special attention has also been given to possible loops in the paths between the driver outputs of the IC and the gates of the MOSFETs. These loops must be kept as small as possible. The loop in the drive circuit for the upper MOSFET of each channel consists of the HO driver, the gate resistor, the gate-source capacitance and the driver return connection (HOICOM or H02COM). For the lower MOSFET, the loop consists of the LO driver, the gate resistor, the gate-source capacitance and the driver ground connection (LOICOM or L02COM).
The overshoots arising from switching the MOSFETs are limited by the resistors incorporated in the gate circuits. This unavoidably leads to a compromise between the delays for switching off one MOSFET and switching on the other one. For this amplifier, limiting resistors with a value of 5.6 Q are recommended. This also allows part of the power that would otherwise be dissipated in the driver transistors to be dissipated in the resistors.
Another compromise is naturally the maximum power rating of the amplifier and the resulting MOSFET selection. Unfortunately, maximum drain current goes hand in hand with high gate-source capacitance (C;S5 = 3800 pF max.). The parasitic self-inductance of the gate is also a factor; the lower it is, the faster the gate charge can be built up or removed.
To accelerate MOSFET switch-off, diodes are placed in parallel with the gate resistors. These are also ultra-fast-recovery diodes in ’normal’ packages (MUR120). Thanks to their relatively large dimensions, they can easily bridge several broad tracks on the printed circuit board. A disadvantage of using these diodes is that the power dissipation in the drivers increases.
Due to their power dissipation, the gate resistors are 1-watt types. We have selected the very compact PR-01 series from BCComponents. Due to the construction of these metallic-film resistors (helical groove), they unavoidably have significant parasitic self-induction, but the impedance of this resistor type is still relatively constant up to 10 MHz. A good alternative would be carbon compound resistors, which have much lower self-inductance due to their construction. Sufficient room is provided on. The circuit board for the latter type.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/04/Clarity_2x300W_Class_T_Amplifier_Part_2__building_amplifier_board__(2).html
Print this Page | Comments | Reading(746)
Author:Ecco Reading(30177)
Author:Ecco Reading(3461)
Author:Ecco Reading(3181)
Author:Ecco Reading(3661)
Author:Ecco Reading(5231)
Author:Ecco Reading(3248)
Author:Ecco Reading(3390)
Author:Ecco Reading(3530)
Author:Ecco Reading(3935)
Author:Ecco Reading(3707)
Author:Ecco Reading(3677)
Author:Ecco Reading(3714)
Author:Ecco Reading(6028)
Author:Ecco Reading(3692)
Author:Ecco Reading(4478)